This is the document that is used throughout the negotiation process. Every time there is a change made during the negotiations, it is recorded on the contract and signed by both parties (buyers and sellers). Most negotiations are fairly easy going; however there are some exceptions, ex: inspections, FSBO's, etc... Once an agreement, that is acceptable to both parties, is negotiated a copy will be given to those involved. See:
http://www.realestateabc.com/homeguide/buying8.htm
http://www.realestateabc.com/homeguide/buying9.htm