Most offers to buy a house are accompanied by a check. This check is generally referred to as the "earnest money deposit." The basic reason for the deposit is to impress the seller that the buyer "earnestly" intends to purchase the property.
The amount of the deposit varies from purchase to purchase, depending on a variety of factors. If a property generates a lot of interest, a buyer may make a larger deposit to convince the seller that their offer is stronger than the others. During "hot" markets, deposits are generally larger than during slow markets.