Beer wouldn't be beer without them (with a few exceptions). Hops come from all over the world and their region generally influences the characteristics. For example, US hops are known for their citrus notes, British hops for their earthy character and 'New World' (NZ and AUS) for fruity, tropical flavours. European hops are often delicately floral and commonly used in Pilsners and Lagers.
The Alpha Acid (%) of a hop is a big factor to consider when determining the hop profile of your beer. A high alpha (10%+) is very bitter and should be used sparingly whilst compared to lower alpha (<7%) which lend more aromatic than bittering qualities to the brew (although if you use enough, you will still get some bitterness out of them). The overlap and middle between these Alpha Acid levels belongs to the varieties of hops considered 'Dual-Purpose' - they are good for both bittering and aroma.