What constitutes a contract with a realtor?

I know I’m sounding more and more ignorant as I go along but…there is no cure for that except to ask questions.

In my question about renting our house the subject of a contract with our realtor came up several times. It’s been over a year since we started trying to sell our house, and I can’t remember if we signed a separate contract especially for our realtor….or (and this is just occurring to me) if the contract we signed with the sellers of the house we want to buy constitutes a contract with our realtor at the same time…?