What is the difference between an Offer to Purchase and a Purchase Agreement?

An offer to purchase is the document used at the time an offer is made by the buyer and before the due diligence period. The definitive purchase agreement is the document signed at the closing and after due diligence. Sometimes, the offer is used as the definitive purchase agreement and sometimes, a separate document is prepared to be signed at closing, which incorporates any additional agreements made after the initial offer was accepted.