This article isn’t going to talk about “different kinds of contracts” in the sense of sales contracts vs. employment contracts vs. real estate contracts. Instead, I’ll be talking about express, implied, and quasi-contracts.
Express contracts are the contracts that parties affirmatively intend to enter into. When you go to buy a car, you sign a contract that is an agreement to purchase the vehicle, so that you and the dealer enter into an express contract for the purchase and sale of the car. But not all express contracts need be written. Oral contracts, generally speaking, are just as effective a form of express contract as printing out a written agreement and signing on the dotted line. If I stop by your house and offer to paint it for $1000, and you agree to have me paint it for $1000, we have entered into an express oral contract.
However, while oral contracts are generally equally as enforceable as written contracts, there are certain types of agreements that must have a signed writing in order to be enforceable — no oral contracts. This doctrine is known as the “statute of frauds”; although the exact list of agreements that must have a signed writing may vary from state to state, the types of contracts that are typically covered by the statue of frauds include contracts for the sale and purchase of an interest in real estate, contracts for the sale of goods over $500, and contracts that are intended to definitely last longer than one year (if the contract is of indefinite duration or it is not definite that the contract will last longer than a year, this “one year rule” may not apply).
The next kind of contract is the implied contract. Implied contracts arise from the conduct of two or more parties that demonstrates that they intend to be bound by some sort of agreement, although no express written or oral agreement exists. For example, let’s say I come by your house and mow your lawn, and you pay me $50. I then keep coming back every two weeks to mow again, and you keep paying me $50 each time. As a result, we may have an implied contract for me to mow your lawn every 2 weeks for $50 every time I mow. Implied contracts are just as enforceable as express contracts, although it is often more difficult to discern the terms of an implied contract since they arise from the parties’ conduct, rather than having express terms written or spoken between the parties.
The final kind of contract technically isn’t a contract at all. It is a legal doctrine known as a “quasi-contract”, and is a form of equitable relief used by courts to avoid unjust enrichment by a party, when that party has something of value conferred upon them by another party in the absence of an express or implied contract.