When you purchase a house, the seller's conveyancer will send a draft contract to your conveyancer for review. Put simply, this is a document that states the terms you'll agree to and the how much you will pay for the property. Your conveyancer will liaise with you regarding the contract, negotiating any changes if required, until you and the seller are happy with a final version.
At this point, you and the seller will have identical copies of the final contract to sign. What comes next is the pivotal 'exchange of contracts'! Once both conveyancers swap or exchange these signed contracts, then both you and the seller are legally committed to the sale and neither can pull out of the purchase. Exciting, I think you'll agree!