In Ontario, Realtors use a document called a Buyer Representation Agreement to formalize the brokerage/client relationship. This document will contain the people who are the buyers, the name of the real estate brokerage, term (length) of the agreement, location where the property search will be (eg: ontario, toronto, cottage country) and how the brokerage will be getting paid by the buyer (if at all).
You may want to pay special attention to the length of the agreement --anything over 6mos commitment needs an ADDITIONAL initial from the client to approve --as 6mos is a long time to commit to an EXCLUSIVE relationship with any individual. --also some people will sign a Buyer Rep Agreement with an agent in one area (Muskoka with me :), and with another agent in Toronto.--it makes sense that nobody can be an expert in all locations, and serve you well in all areas of your search.
Like all contracts, you need to read it through, ask for clarity if needed, and your realtor should explain it thoroughly to you, without any pressure to sign. The 'devil is in the details' --make sure you know what you're committing to, and move forward confidently with the realtor you choose.
