Agreement to Purchase vs. Offer to Purchase

Hi: And my first question out of the box as a BNG is what is the difference between "Agreement to Purchase" and an "Offer to Purchase"? Everyone tells me to get those offers out there. OK, fine. What document do I use to submit those offers? I found the "Agreement to Purchase" document, but do I use the same document for an "Offer to Purchase"? It seems to me that these are two difference topics. Am I wrong? Any assistance would be greatly appreciated.
Thank you.
AW