Sponsorship agreements can be as simple as a verbal agreement to give a donation of a hundred bread rolls from the local bakery through to complex five-year agreements for naming rights of your local competition.
It is good business practice to have a formal agreement or contract in place and is a necessity for significant funding. Lack of a written contract increases the potential for misunderstandings and the relationship turning sour. A change of personnel can mean the intent of an agreement is lost, unless the agreement is in writing.
If unsure, consult with a lawyer to ensure the agreement is sound.
Never assume you will get certain rights. Clearly state the rights agreed to or commitment in the contract.