There are two types of work permits given by Canadian authorities- open work permit and an employer specific work permit. An open work permit basically allows you to work for any employer. This visa is not job-specific, so applicants do not require the Labor Market Impact Assessment (LMIA) or an offer letter from an employer who has paid the compliance fee.
With an open work permit you can work for any employer in Canada except for those companies that do not comply with labor requirements or are involved in services like escort services, erotic massage or exotic dancing.
The employer specific work permit as the name suggests is a permit that allows you to work for a specific employer.