At its most basic level, a written agreement is a agreement that is a legally enforceable agreement between two or more parties (individuals, businesses, or organizations) that creates an obligation to do, or not do, certain things.
Regardless of whom the parties are, agreements almost always contain the elements of who, what, and how.
The "who" means the parties must be competent to enter into an agreement.
For example, under the law, a minor generally cannot enter into a contract.
The "what" refers to the mutual agreement of all parties.
Each party either promises to do or not do something.
"What" is it that you are supposed to do and "how" are you going to do it?
For example, if you enter into an agreement to buy a car for $10,000, the "what" is buying the car and the "how" is the $10,000 payment.