Taken from the Hunter College, City University of New York (CUNY) website

So, you think you want to major in Psychology? The subject matter of psychology is very broad. There is an astoundingly diverse range of issues with which psychologists are concerned and settings in which psychologists work. Let's look at the various subfields of psychology, keeping in mind that within each subfield, psychologists can engage in many different activities. Most conduct research, teach, and/or apply psychological principles to the solution of the real life problems of individuals and society.

Biopsychologists take a comparative and ontogenetic perspective in the experimental analysis of basic psychological processes as they relate to the many ways in which animal species adapt, survive, reproduce and evolve. 

Child Psychologists: See Child Clinical Psychologists, Developmental Psychologists, School Psychologists, and Social Worker. 

Child Clinical Psychologists have about the same preparation as clinical psychologists; in addition, they have the responsibility of becoming acquainted with the developmental characteristics of preadolescent. 

Clinical Psychologists are concerned with the diagnosis and treatment of psychological disturbances. After graduate preparations in an accredited university or school of professional psychology, supervised postdoctoral experience, and licensure or certification, some clinical psychologists enter independent practice/consulting roles. Others find themselves responsible for a complete range of psychological services in public settings. Their responsibilities range from administering and scoring psychological tests, to engaging in therapy, to supervising the training of graduate students in the delivery of mental health services, to administering community mental health programmes. Some clinical psychologists obtain faculty positions in a college or university where they perform research and train graduate students. Others serve as adjunct (or part-time) faculty, while maintaining independent clinical practices. Many serve as consultants. Ph.D. or Psy.D. needed.

Cognitive Psychologists are concerned with what organisms know and how they come to know it. They study how people perceive objects and events, how they can store past events in memory, how they transform and manipulate knowledge by thinking, and how they analyze their experience to emerge with new and abstract notions.

Community Psychologists are concerned with the application of concepts, tools and skills from the fields of counseling, guidance and rehabilitation psychology. Community psychologists are trained to counsel, treat and/or refer outpatients and to assist in the development of community resources for their clients. 

Counseling Psychologists are concerned with counseling, teaching, consulting research, and/or administration. In their work, they are particularly concerned with the role of education and work in an individual's functioning, and with the interaction between individuals and the environments in which they live. Typically, counseling psychologists work with normal or moderately maladjusted persons, individually or in groups. This work includes use of traditional counseling interview methods, interest, ability and personality tests, and educational and occupational information. Most counseling psychologists are employed in educational settings and most of those in colleges or universities. A minority are employed in hospitals, community mental health or rehabilitation centers, industry, government, or in private or community counseling agencies. 

Developmental Psychologists are concerned with growth and development from conception through death. All aspects of the animal or human organism (physiological, biological, physical, cognitive, emotional, social, cultural) may be studied. 

Educational Psychologists are concerned with a range of activities from initial design through development and evaluation of both materials and procedures for education and training. Such positions exist in public schools, in the military, in private research and development companies, and in industrial concerns. They may deal with analyzing education and training needs, with developing materials for instruction in various media, with designing the best conditions for instruction, and with evaluating the effectiveness of instructional programs. 

Engineering Psychologists are concerned with designing and using environments and systems with which human beings live and work. Their job is to improve the interaction between people and their environments. They may help design work areas, equipment, and human-machine systems, as well as the training devices, aids, and requirements needed to prepare people to make such systems function. They work in many different settings, such as the aerospace industry, communications and transportation industries, the military, and other governmental, commercial and industrial enterprises.

Experimental Psychologists: A general title applied to a variety of psychologists who are trained in designing and conducting research in specific basic areas like learning, sensation and perception, human performance, and motivation and emotion. A research oriented doctoral degree (Ph.D.) is usually needed. 

Health or Medical Psychologists are concerned with understanding how psychological factors can affect physical health. Some representative issues: how certain attitudes and behaviors may cause heart disease; how the state of physical health affects one's psychological well-being; and how living with cancer can affect one's attitude and behavior. They also study the psychology of how people can better cope with physical disease. 

Industrial/Organizational Psychologists are concerned with the relation between individuals and work. They are employed in business and industry, in government, and in colleges and universities, and may perform a variety of jobs. An industrial/organizational psychologist working in industry may study how work is organized; suggest changes to improve the satisfaction of employees, the quality of the organization's services, and productivity; consult with management on the development of effective training programmes for employees; design programmes for the early identification of management potential; administer career counseling and pre-retirement counseling programmes; develop affirmative action programs; recommend changes in job definition; design a system of performance evaluation. A master's degree in industrial/organizational psychology is a minimum requirement for a position as a psychological associate but a position as a psychologist requires a Ph.D. 

Personnel Psychologists: Compared to industrial/organizational psychologists, personnel psychologists focus more specifically on the hiring, assignment, and promotion of employees. Such a psychologist may be involved in the continued development and validation of assessment tools for selection, placement, classification and promotion of employees. Although positions as test administrators and interviewers are available for those with B.A.'s and M.A.'s, a Ph.D. is a general prerequisite for a position as a psychologist. 

Psycholinguists are concerned with discovering the psychological significance of the properties of language, of linguistic organization, the meaning of words, syntax and how children acquire language. 

Psychometric (Quantitative) Psychologists are directly concerned with the measurement of behaviour (mostly through the use of tests) and the design of research investigations. Such a psychologist may be analyzing complex sets of data; designing, developing pilot testing, or validating versions of intelligence, personality, or aptitude tests; or deriving new statistics with which to analyze data. The psychometric psychologist is typically well-trained in mathematics, statistics, and in the programming and use of computers. 

Psychotherapists: A very broad term. Generally, anyone can call him or herself a "Psychotherapist", but ONLY a person with a Ph.D. in Psychology and a license can use the term "Psychologist". Psychotherapists can include anyone without a Ph.D. who tries to help people deal with their problems. Counselors and Social Workers are psychotherapists, but not Psychologists. 

School Psychologists are concerned with increasing the effectiveness of schools in improving the intellectual, social and emotional development of children. They may also serve as consultants in education for the handicapped, mentally disturbed or mentally retarded; or in developing special programs in adult education. They typically function in various roles within the school system. They may assist in implementing and evaluating special education programs, may serve as leaders of in-service training programs for teachers, or as consultants to teachers regarding specific teaching or classroom related problems. They may also help to treat children's psychological and educational problems that influence problems in school. They may also administer tests and interpret their results.

Social Psychologists study the ways in which the social context affects the behavior of the individual and groups in the real world and the laboratory. Social psychologists focus on topics such as social roles, attitude formation and change, affiliation, interpersonal attraction and interaction, conformity, and group processes. 

Social Workers attempt to meet social needs by providing leadership in the development of social policy and practice in the field of social welfare. Social Workers help advance the professional practice of social work by enhancing the social functioning of individuals, groups, families, and communities. Two subfields exists in the field of Social Welfare, such as specializations in either direct practice, or management concentrations. 
 

To give you an idea of how varied the interests of psychologists are, here is a list of Divisions (interest groups) of the American Psychological Association. Why not visit the website of the ones in which you're interested for information about careers, study, etc. Many of the Divisions also have listserves in which you can participate whether or not you are a member.  
   

Division 1 - General Psychology  

Division 2 - Society for the Teaching of Psychology  

Division 3 - Experimental Psychology  

Division 5 - Evaluation, Measurement, and Statistics  

Division 6 - Behavioral Neuroscience and Comparative Psychology  

Division 7 - Developmental Psychology 

Division 8 - The Society of Personality and Social Psychology  

Division 9 - Society for the Psychological Study of Social Issues (SPSSI) 

Division 10 - Psychology and the Arts  

Division 12 - Clinical Psychology 

Division 13 - Consulting Psychology 

Division 14 - The Society for Industrial and Organizational Psychology  

Division 15 - Educational Psychology 

Division 16 - School Psychology 

Division 17 - Counseling Psychology  

Division 18 - Psychologists in Public Service  

Division 19 - Military Psychology  

Division 20 - Adult Development and Aging  

Division 21 - Applied Experimental and Engineering Psychology 

Division 22 - Rehabilitation Psychology  

Division 24 - Theoretical and Philosophical Psychology  

Division 25 - Experimental Analysis of Behaviour  

Division 26 - History of Psychology  

Division 27 - Society for Community Research and Action: Division of Community Psychology  

Division 28 - Psychopharmacology and Substance Abuse  

Division 29 - Psychotherapy  

Division 30 - Psychological Hypnosis  

Division 31 - State Psychological Association Affairs 

Division 32 - Humanistic Psychology  

Division 33 - Mental Retardation and Developmental Disabilities  

Division 34 - Population and Environmental Psychology  

Division 35 - Psychology of Women  

Division 36 - Psychology of Religion  

Division 37 - Child, Youth, and Family Services  

Division 38 - Health Psychology  

Division 39 - Psychoanalysis 

Division 40 - Clinical Neuropsychology 

Division 41- The American Psychology-Law Society 

Division 42 - Psychologists in Independent Practice 

Division 43 - Family Psychology 

Division 44 - Society for the Psychological Study of Lesbian, Gay, and Bisexual Issues  

Division 45 - Society for the Psychological Study of Ethnic Minority Issues 

Division 46 - Media Psychology  

Division 47 - Exercise and Sport Psychology  

Division 48 - Peace Psychology  

Division 49 - Group Psychology and Group Psychotherapy 

Division 50 - Addictions 

Division 51 - The Society for the Psychological Study of Men and Masculinity 

Division 52 - Division of International Psychology