In Canada, a major (also known as a major concentration or concentration) is a term for the specific group of courses that give you a basic knowledge of a field of study, which is in addition to your core curriculum. A major is a term most commonly applied to a program of study leading to a bachelor's degree. The university, college, school or program department will define a framework for this specialized portion of your studies, including a certain number of required courses and a certain number of "elective" courses relevant to the major. The school will also define your general education or foundation education requirements. Some majors effectively define your full course of study; many others allow you considerable latitude both within your field of study and in their other courses. However, not all schools in Canada have majors and may offer a program of "general humanities" or "general liberal arts" studies, which is, in fact, a major of a different kind.
Although most international students choose their major before entering a university or college, many others will select it during their first or second year of a three- or four-year program. Many humanities and liberal arts schools or program departments do not allow you to officially declare your major until the end of your second year so that you experience a broad range of courses that will help you choose the most appropriate major. Some schools actually require international students to list a major choice on your application for admission, but this is rare. If you are interested in a major that requires a lot of classes, or classes that are limited to students in that major, you might have to declare earlier than usual (i.e., the fall semester of your second year). As well, for some majors you will need to take specific courses (pre-requisites) during your first and second year before you can even be considered eligible for upper level courses. Some humanities and liberal arts majors have limited enrolment so in fact you may even be required to apply to get in to the major you want, including attending an interview or writing an essay.
Community and career college programs in Canada generally do not have majors as such. Their 1- to 3-year diploma and certificate programs carry a designation, for instance Certificate in Aboriginal Justice Studies, which is much more specific than, say, Bachelor of Arts. Often, international students in a community college or career college program--sometimes all the students in the department--will take a common set of core foundational courses in the first year (or term, in the case of shorter programs) which then enables them to pursue a specialized area of study. But generally the field of study at the community or career college level in Canada is job-focused to the point where students do not take "elective" courses as they do for bachelor's degrees. Hence the designation of the diploma effectively stands in for the "major."