A merger clause is a clause in a contract that states the written agreement represents the complete agreement between the parties. These can be worded several different ways, such as: "This contract is intended by the parties to be the full and final expression of their agreement."
If you sign a contract with a merger clause, that means all previous promises made during negotiations will not be enforced unless they are included in the written agreement. Even if the prior promises were in writing (for example, in an e-mail or fax), those specific promises must also be included in the contract in order to be enforceable.
Merger clauses are a good example of why it's important to have an attorney review your agreements before you sign them.