What are the main sections of a written contract?

There are 3 key principles which make a contract valid.

1. Offer - a good or a service needs to be offered.

2. Acceptance - the buyer must be willing to have what is being offered

3. Consideration - the amount the buyer is willing to give in exchange for the good or service in the offer.

Other secondary principles are often mentioned, such as ability to contact i.e. being of legal age, sound mind etc.

There is no necessity for a contract to be written to make it legal. If the 3 conditions exist, then the contract is legal. However, in cases of potential dispute, it is always easier to have a written copy, signed by all parties confirming the above principles.