The hop plant (Humulus lupulus) is a plant whose flower cones ('hops') are used in the production of beer. In Britain, for the last 500 or so years they have been used to provide bitterness in beer to counter the sweetness of the malt, and they also provide useful preservative qualities.
Prior to hops being introduced to Britain, herbs were used ('beer' in those days was unhopped, and 'ale' was hopped - these days the terms are interchangeable)
There are many types of hops and they each have their own aromatic and flavouring qualties. By combining these at different times in the brew, the flavour can be massively changed.
Hops come in three main forms:
In my first brew (a kit beer) I used hop powder as it came with this, which was made from powdered Goldings hops.
- Whole-leaf hop flowers - these come vacuum packed. Use quickly when opened - failing that, store in dark cold loations such as a fridge.
- Hop pellets - They're made from pulverised hop flowers. Very space efficient but can oxidise quickly if not vacuum-sealed.
- Hop plugs - Compressed hop flowers - convenient and the processing incorporates more of the freshness of hops than with pellets.
- Hop extracts - these can be oils or powders and are primarily of use to those starting up in homebrew. You add the powder / oil and that's all there is to it.
In a brew, you can throw in hops and leave it or get very intricate with a number of hops, each added into the mix at the time of your choosing to obtain unique results. A very true maxim I heard about brewing is that it's like baking bread - it uses a finite number of ingredients, but with careful combination and preparation you can achieve utterly unique results.