As defined in Lee's Brewery Glossary page, a hop is vine whose flowers are used as a bittering agent in beer.
Hops also contribute to the beer's aroma and can even increase the beer's shelf life.
Lee's Brewery preferred hop form is the pellet. Pellets are easier to handle as well as weigh than leaf hops. Hop plugs are also easy to handle, but their standard 1 ounce size makes them impractical for small recipe additions. Both pellet and plug hops store much better than leaf. Of course if you're dry hopping your beer, then you must use leaf hops.