What is the difference between an exchange of contracts and completion?

An exchange is when the buyer and seller enter into a legally binding agreement to buy and sell on a set date. Once an exchange has taken place, both parties are committed to the sale/purchase, and cannot pull out of the transaction without legal penalties. Exchange of contracts is also when the buyer must pay a  deposit, usually 10%. 

Completion refers to the day on which all payments have been made/received, that the property is transferred from the seller to the buyer; the day that you move house