A single-entry visa allows you to enter Canada for only one time while a multiple entry visa allows you to enter the country for a multiple number of times.
Generally, all visa applicants are considered for a multiple entry visa and a visa is issued after completing the review process.
With a multiple entry visa, you can travel to Canada for a six-month period every time and as many times as you want. The multiple entry visa is valid for 10 months or one month before your passport expires whichever happens earlier.
With a single-entry visa you can travel to Canada only once. You are eligible for a single entry visa if you are on an official visit or you intend to participate in an event in Canada.