I've just taken a few statements out of the Wiki and put this in.
I hope that has clarified it a bit.
Ale is a generic term that does not describe any particular style of beer but a wide range of styles. Bitter is a bit more focused. Real ale can include Pilsners.People visiting the UK are often confused by the terms Ale and Bitter.
Bitter generally is a wide all encompassing term for beer that is not Lager, Cider, Stout, Brown Ale or Mild.
Ale is a term that can include all but Cider and Lager and includes bitters, brown ales, porters. If you just ask for an Ale you'll get pot luck or a strange look. If you ask for a bitter you generally get the house best bitter.
You may be better asking for a real Ale. Which can cover anything from a Hand pump apart from Cider.
I hope that has clarified it a bit.