The Bitter style came from brewers who wanted to differentiate these ales from other mild brews, enter pale malts and more hops. Most are gold to copper in colour and are light bodied. Low carbonation. Alcohol should be low and not perceived. Hop bitterness is moderate to assertive. Most have a fruitiness in the aroma and flavor, diacetyl can also be present. These are traditionally served cask conditioned, but many breweries have bottled versions.
ESBs are essentially more aggressive and more balanced Bitters, both in alcohol and hop character, but nothing overpowering. Color range will be similar, though leaning towards the darker end of the scale; dark golds to copper. Low carbonation. Malts tend to be more pronounced, often toasty and fruity, with maybe some notes diacetyl. And despite “bitter” being in its name, ESBs are not really all that bitter. They key to an ESB is balance. (BeerAdvocate)