Any type of temporary permits such as visitor, study, or work permits can be extended once the applicant is inside Canada. There must be genuine reasons to apply for an extension and it should be applied for well before the status expires i.e. before the applicant starts to overstay. It is not advisable to keep it to the last minute to apply for an extension. If an extension application is submitted, the applicant is said to have 'implied status' till the decision on the application comes.
Restoration is the application to reinstate the status after the applicant has overstayed for less than three months. Justifiable reasons for overstay need to be mentioned in the application and efforts should be made to reinstate the status even if you do not want to stay in Canada anymore. If you want to change your status, restoration and change of status applications are submitted together