A bribe is a bribe. A contracted reward is not a bribe. When you reward a child for doing something you have agreed upon ahead of time, or for something he/she does that is expected and understood, you are rewarding, praising and reinforcing the behavior you want. This is not a bribe; there is nothing wrong with this logical consequences strategy.
When you reward a child who is misbehaving during the misbehavior in order to stop the poor behavior, you are bribing the child. In the bribe situation, the child quickly sees that the way to get another treat (bribe) is to misbehave again.
Contracting rewards good behavior, behavior the child understands and knows that you want. Bribing rewards bad behavior and encourages it to continue.
Parents usually bribe out of desperation; contracting is planned. Contracting is a positive guidance and discipline strategy. I think of it as "Grandma's Rule": Do the chores well first; then you can have the cookie.