Three main elements are needed for a contract to be enforceable, they are;
Offer: Which must be clearly stated and presented to the offeree, this can be in either an oral or written form.
Acceptance: Acceptance must be acceptance for the exact offer which was stated, any deviation may count as a counter offer and lead to the contract being unenforceable
Consideration: Something of value given by both parties to a contract that induces them to enter into the agreement to exchange mutual performances.
There are also other elements to a legally binding contract such as expressed and implied terms of the contract. Implied terms can consist of terms implied by law e.g The Statute of Frauds requires the sale of land to be in writing etc, these laws change from jurisdiction to jurisdiction so you would have to contact an attorney in your area to find out more. For a contract to be concluded, performance must be followed down to the letter of what was contracted for, courts give very little leniency in this area.
It's also worth checking out the law surrounding Duress, Misrepresentation and Undue Influence to fully cover yourself in the event of a breach by either party.
Of course, the above answer is only a very brief outline of the elements of a contract, If you're asking for a reason other than personel intrigue I would strongly suggest you ask an attorney in your locality. Otherwise, I hope the above helps.