A contract is an agreement, usually written out on a legal document, in which it states terms and agreements, as well as rewards that would be given at the end when the terms are met.
For example, a contract for assassination may be posted out in the black market. An assassin would then agree to take on the requirement of the contract. If it is met, the payment would be given to the assassin. If not, then the contract becomes void. This makes contracts important because the terms and everything is written down, and can be looked on for future references.
