Glossary
Contract
A legally enforceable agreement between two or more parties. Most contracts don't have to be written down, but some (like real estate transactions) do under each state's "statute of frauds."
A contract requires offer, acceptance, and consideration (something of value being exchanged). All three have to be present or the agreement isn't legally enforceable.
Most everyday transactions create contracts whether the parties realize it or not: buying a coffee, signing a lease, agreeing to terms of service. The complexity comes when something goes wrong and one party claims the other didn't perform what they promised.
Contract law is mostly state law, varying significantly between jurisdictions. The Uniform Commercial Code (UCC) governs most sales-of-goods contracts and is fairly uniform across states.