Letter of introduction:
This is issued when you get your visa approved. This letter contains your permit reference number and the CBSA officer needs this letter to issue your study permit.
Study permit:
This is the official document issued by an officer that allows a person who is not a Canadian citizen or a permanent resident to study in Canada.
Study visa:
This is a temporary resident visa which is an official document issued by an officer that is placed in your passport to show that you have met the requirements for admission to Canada as a temporary resident.
When your application for Study permit approves, Your passport is sent back to you with Student visa stamped on it. while your study permit is not sent to you at this time instead they send you Letter of Introduction. Which is required at the port of entry. when you show the letter of introduction at the POE to the officials, the officer grant you your study permit then.
Visa is required to enter canda
Study permit is required to study in canada