Once you submit your offer the seller has a certain amount of time as defined in the contract to get back to you. They well either accept the offer as is, completely reject it and not even counter (this may happen if they feel the offer is ridiculously low), or present a counter offer. Typically this is all done through the agents representing the buyers and the sellers.
The counter offer can be a change to the dollar amount, conditions or a combination of the two. For example, you want to buy a home listed at $200,000. You make an offer of $180,000, with mortgage and inspection contingencies and a 90 day closing date. The sellers may come back with a counter of $190,000 and a 60 day closing. You may counter the counter with $185,000 and a 60 day closing. This can go back and forth until both parties can come to an agreement, or if they can't, it's time to move on. NOTE: In the state of Pennsylvania, offers from other parties can be considered and accepted while a negotiation is in progress. I have seen people lose a home while they were in the middle of aggressively negotiating when a better offer came in and the seller accepted it. Keep in mind, that while you were looking for a home in a certain geographic area in specific price range, other were too. In fact, there is a good chance you have looked at many of the same homes. If you decided the one you have made an offer on is the best, they may have come to the same conclusion. Once you find a home you like, be ready to make a reasonable offer, do it quickly, and try not to let your negotiations drag on. If you do, someone else may beat you to it!
Once the terms are agreed upon, the buyers and the sellers sign and initial the contract where required and the contract becomes what is known as a fully executed contract. There are many things that have to be done by both the buyer and the seller between the signing of the contract and the closing date. Theses items are defined in the contract as well as the time frame in which to complete them. Typically the time is a certain number of days from the execute date. The next section "Accepted Offer" will further explain these tasks and provide you with a tool to assist you with staying on track and meeting your deadlines.