What is a principle of contract law?

There are basically two, main concepts and principles to contract law:

First, being mutual consideration. BOTH parties have to receive benefit, or its not a legal contract. If only one receives benefit, then there is not a contract.

Second, there has to be a "meeting of the minds." This means, both understand what they are agreeing upon. Both are agreeing on the same things. If one thought it meant something totally different, then there was not a meeting of the minds and the contract is void.