In order to legally work in Canada, you will need either a valid Social Insurance Number (SIN) or a Work Permit. Many international students in Canada obtain Study Permits which allow them to work on-campus for the institution they study at. In most cases, working on campus does not require a Work Permit.
Working off-campus is different, as foreign students do, in fact, require a Work Permit, except in the cases of work-study placements such as internships and co-ops. To obtain a Work Permit, students must have full-time enrolment at a public institution or, an approved program at an eligible private institution. A Work Permit authorizes students to work for up to 20 hours a week during academic session and full-time hours during scheduled breaks. Maintaining your Work Permit will require that you remain a full-time student and maintain adequate academic standing.
If you are an international student seeking employment in Canada after graduation, you must also apply for a Post-Graduate Work Permit which tends to vary in length. If a student has completed a minimum of two years of education in Canada, they may be eligible for a Work Permit of up to 3 years. In cases where the international student has spent less than 2 years but longer than 8 months studying in Canada, they are eligible for a Work Permit for the same duration as their period of study. Cases tend to vary so it is important to do your research in advance of seeking employment.