A
major is a field of study at the
undergraduate level, and represents competence and familiarity with the material, history, background, skills, methodology, and practice of the subject in question. Undergraduate
degrees typically note which major(s) a graduate has completed. The attainment of a major generally requires the completion of a significant number of classes dealing with the subject area, typically including several classes consisting of the "
core" of the major which cover introductory, basic, and fundamental aspects of the field, which all students in a particular major will take, in addition to several more targeted classes dealing with more specific aspects or areas of study of the field in greater detail, with students typically free to choose from among many such specialized courses.
Most colleges require students seeking Associate's or Bachelor's degrees to choose and complete a major. However, some colleges offer general majors, with names like "Undergraduate Studies", "Liberal Arts", or "Arts and Letters", which typically represent a diverse education with more of a focus on research skills, structure, and logic, than on any particular subject matter, and some schools like St. John's College, with its "great books" curriculum, operate outside of a "major" paradigm.
Majors are usually affiliated with and administered by the academic department which offers courses in the field, although some departments may oversee multiple majors and some majors may be conducted as cross-disciplinary programs. Majors can range from the abstract and academic to concrete, skills-based career preparation, and many colleges offer majors specifically designed as precursors to further professional education (law, business, or medical), although given adequate preparation it is certainly possible to attend such schools regardless of one's undergraduate major. Most graduate students major in the field they will go on to study, but this is less a matter of cause and effect - you don't major in anthropology so you can study the subject in graduate school, you major and then go for your PhD because you like anthropology.
In addition to the requirements for a major most colleges require students to complete either a common "core curriculum", which may either consist of an actual series of prescribed classes, a selection of "Chinese menu"-style classes ("Complete two courses from Group A, three from Group B..."), or simply broad "distributive" requirements ("students must take one course in mathematics, two in the physical sciences, one covering a period of history prior to 1800..."). Courses taken that fulfill neither major nor these general requirements are usually considered "electives", reflecting their "optional" nature.
Some students, if the option is available, may complete, in addition to a major, one or more minors. A minor is kind of a "lite" major - a student pursuing a given minor will have to take fewer classes than students majoring in the same field, and typically has fewer requirements on those he or she does take (required grade point average, mandatory sequences, and the like). Minors represent a focus on a field to the extent that a student has significant familiarity with the subject matter, though not with as many specifics or in as much depth as a major. Minors, as with majors, may go by other names at individual institutions, even if they represent fundamentally the same concept. Both have been called "concentrations" at various colleges I am familiar with.
Further, some students can and will "double major", that is to say, complete the requirements for two majors and be recognized for both. Triple- and greater multiple-majors are very rare but not unknown; students pursuing this course will tend to have very heavy workloads, few opportunities to pursue electives, and may take longer than the standard 4 years to complete their undergraduate education.