Temporary work permits can be granted depending on the occupation and a job offer. Some jobs do not require a work permit but must meet certain criteria. These include competing sports people, business visitors, convention organisers, clergy, emergency service providers, foreign representatives, media crew etc). Most others require a work permit.
Most temporary work permits require a Labour Market Opinion (LMO) – an official document showing that it is an occupation that can be filled by a foreigner.
Some people may be granted an open work permit, such as eligible spouses of a skilled worker, or those who have been approved for permanent residence under another category.
For more information about work permits (eg Federal Skilled Workers), see Canada Migration.