Bitter is a traditional British style. In order of increasing ABV (although there is a lot of overlap) there is: ordinary bitter; special bitter; and extra special bitter (ESB). English IPAs are sort of a fourth category, being more like a stronger, slightly hoppier ESB than an American hop-bomb pale ale.
Traditionally, the correct way to serve bitter is naturally carbonated from a cask, either by gravity of by a beer engine. The nitro widget is a new fangled thingy, and is certainly not required. If you bottle, just prime as normal. Or keg. Or look into a real beer engine...
ETA - BJCP Entry on English Pale Ale