Dry hopping is when you add hops directly to your fermentor or keg. Usually the hops are added to the secondary fermentor to minimize the risk of contamination because the hops will not be boiled like your beer was. By the time the beer is in the secondary fermentor the risk of bacterial infection is less due to the yeast, alcohol and lack of oxigen. Dry hopping does not add bitterness to your beer, just great flavor and aroma. I usually like to keep the dry hops in my secondary fermentor for about 2 weeks just before bottling. It is said that hops have a natural antibacterial property so just pitch them right in out of the package (but try not to let them touch any unclean surface). If you like the smell and taste of hops, then pick your favorite and throw it in there.