Open work permit
An open work permit lets you work for any employer in Canada, and you do not need a positive LMIA or a job offer to apply. You can only get an open work permit in specific situations as following:
Employer-specific Work Permit
An employer-specific work permit lets you work in Canada according to the conditions on your work permit. Before you submit your application for an employer-specific work permit, the Canadian employer who wants to hire you must complete certain steps and give you either a copy of a Labour Market Impact Assessment or an offer of employment number to include in your application.
The process of applying for a Labour Market Impact Assessment is rather complex, please refer to the IRCC's website for more information.
 In some cases, you can work without a permit in Canada, refer to the Immigration and Refugee Protection Act (S.C. 2001, c. 27) & Immigration and Refugee Protection Regulations (SOR/2002-227) for legal requirements.