Simply stated, a contract is formed when there is an offer and acceptance of that offer. The actual written contract is never quite so simple.
The written contract will, of course, contain the specific terms and conditions of the parties' agreement. Once signed, the contract is binding and, by the appearance of your signature, it will be presumed that (i) you know what is in the contract; (ii) all of the terms and conditions of the contract accurately reflect your agreement with the other party; and (iii) the contract is complete, on its face.
Contracts usually also contain several 'boilerplate' provisions, however. Unaware of the pitfalls that such boilerplate conditions can present, many people only pay attention to the specific terms and conditions of their agreement assuming, incorrectly, that the boilerplate contract terms do not matter. Consider, however, the following standard contract provision:
"This contract contains the entire agreement of the parties with respect to the subject matter hereof and supersedes all other previous understandings or contracts, whether oral or in writing, existing between the parties and relating to the subject matter hereof. All agreements between the parties regarding the subject matter hereof are embodied herein and the same is entire in itself, and not a part of any other agreement."
It does not matter that you may have discussed an additional term or condition prior to your signing the agreement; if it does not find its way into the agreement, itself, it will not be considered a term or condition of the contract.
Because this, and boilerplate provisions like it, can have a profound effect in the event of a disagreement between the parties, it is vital that you have the proposed contract reviewed by an attorney before you sign on the dotted line.