A major is a collection of classes revolving around a subject (like mathematics), a theme (like peace studies), or a professional field (like business or engineering).
To graduate from college, you must choose a major and complete a certain number of classes within it. The benefit of a major is that it allows you to get beyond the breadth of standard academic coursework and really engage a specific area of study.
Unlike tarot cards, the major you choose will neither predict nor guarantee your future. Many graduates find jobs that have nothing to do with what they studied in college. According to the U.S. Department of Labor, the average twenty–something switches jobs once every three years and the average person changes career fields two or three times in their lifetime. Have no fear that choosing a college major will lock you into a specific career for the rest of your life.
If you intend to earn a professional degree (like an MD) after college, you will probably need certain courses under your belt. But many future doctors major in non–science related fields.
Preferably before you cross the dais at graduation. Seriously, the answer varies across schools and programs. Some colleges ask you to list your expected major on their application (although "undecided" is usually an option), but don't require you to declare definitively until sophomore or junior year.
If you are interested in a major that requires a lot of classes, or classes that are limited to students in that major, then it is better to declare early. Some majors demand a strictly regimented order of courses, and if you fall behind, you may have to extend your college stay for a semester (or two or four).
Definitely. One of the most exciting facets of college life is that it introduces you to new subjects and fosters new passions. You might enter undergrad enjoying physics but discover a burgeoning love for political science. However, keep this mind: Every major has requisite coursework. Some require you to take introductory courses before you move into the more advanced classes. Also, some classes are offered in the fall but not in the spring, or vice–versa. If you change your major late in the game, it may take more than the traditional four years to earn a degree.
If one field of study doesn't satisfy your intellectual appetite, consider a minor. A minor is similar to a major in that it's an area of academic concentration. The only difference is that a minor does not require as many classes.
Some undergrads with a love of learning and an appetite for punishment choose to pursue two majors, often in totally different subjects. A double major provides you with an understanding of two academic fields. It allows you to become familiar with two sets of values, views and vocabularies. That said, it also requires you to fulfill two sets of requirements and take twice as many required classes. You won't have as many opportunities to experiment or take classes outside those two fields.
While a minor or a double major might make you more marketable, both professionally and for graduate study, both are time—and energy—intensive. Most students find that one major is more than enough.