In legal terms, a contract is any agreement between parties to exchange things of value, such as goods and services for cash. A simple handshake agreement is legally binding, but its terms are not always provable if a dispute arises. However, you don't necessarily need pages upon pages of legalese. A basic contract agreement often works best for a simple business transaction.
You may want to have an attorney review and make suggestions on a template of your business contract. An attorney can pinpoint issues for your particular business that you may need to include in the agreement. You can use a business agreement template and modify it to fit your needs, rather than writing the agreement from scratch.