Bitter also called Pale Ale.
The terms are virtually synonymous now.

Colour ranges from pale golden to copper.
Carbonation is usually quite low.
Hops are mostly in the aroma, these beers are not usually especially bitter to the taste.
Taste is often quite earthy with fruit and berry flavours sometimes there might be some butterscotch like tastes. These beers are usually quite complex and subtle in tastes.
These are session beers and the major type of beer sold in English Pubs, as such they usually come in at 3.0% to 5.0% ABV., usually averaging around 4%.
Bitter or Pale Ale is often "Cask Conditioned" which means that the beer is unpasteurized and the yeast is continuing to work while the beer ages in its keg. Cask conditioned beer has a limited shelf life so drink it promptly.