I am looking to buy a home (my first). My sister just said something about "signing a contract with the realtor" however the realtor I've been in communication with hasn't brought that up. Does one always sign a contract with a realtor? What does it cover? For whose benefit is this, mine or the realtor's? Is this common practice?

I'd like to get some background information on it before discussing it with my realtor, if I can ascertain that this is a valid, common practice,

Why would he not have mentioned this contract to me before?