Can realtors be trusted? What percentage of the sale do they receive?

Please tell me that realtors are not like car salesmen. I must sell my mothers house and one realtor, I have talked to, seems okay. But, I feel he has his own interest at heart and not mine. Its all about the money. If you have had a good or bad experience with a realtor, I need to know what to expect. Question: what percentage of the sale does a realtor receive and is this percentage “set in concrete”? Is there a contract to sign?