Dry-hopping is the addition of hops after fermentation which is like steeping tea. It has nothing to do with bitterness and everything to do with flavor and aroma. Hops are added directly to finished beer instead of being boiled; which is the conventional method.
Cans are the best method for delivering the best tasting beer to you. Recent breakthroughs in canning liners have eliminated the oxidation of aluminum into beer.