If the other party agrees to the offer, acceptance occurs and a contract is made. That acceptance can be made in writing or stated orally. Acceptance can also occur through action. A condition is something that must occur for the contract to be fulfilled. Agreeing to buy a house on the condition the seller replaces the roof is an example.
Contracts can be a simple transaction or a complex, negotiated agreement.
In situations involving businesses or large amounts of money, hiring an attorney to represent you is wise for your best interests.