"Bring me a rock, please."
Ever get the feeling that you are being asked to deliver a product or work that has not been completely defined? You assume you know what a rock looks like based on basic information provided. You find it and paint it, only to find the client wanted a small round rock painted red, not the large square rock stained blue you assumed was needed.
On any project, be it instructional development, system implementation, or web design, it is important that you sit with the client and document the requirements of the end product. You need to document expectations and establish a way for all parties to propose and approve changes to requirements and expectations.
If you don't document requirements and expectations, you run the risk that neither are reached. Or, if they are, you have spent more resources than you needed to meet the requirements and expectations.
Depending on the project, documentation can be managed in different formats, from simple emails to contractual documents. The legality associated with documentation is going to vary so if you need a legal agreement, consult a contract expert.
Bottom line, document, document, document.