Contract
An agreement between two or more people that the law will enforce.
Definition
A contract is a legally binding agreement. For a contract to be enforceable it generally needs an offer, acceptance, consideration (something of value exchanged), and an intention to create legal relations. Contracts can be written or oral, but written contracts are much easier to prove.
In practice
Keep a signed copy of every contract. If something goes wrong, the written terms are the first thing a court will look at.