An Earl (or Jarl) was a title for nobility used by Anglo-Saxons and Scandinavians. It originally meant "chieftain" and it referred especially to chieftains set to rule a territory in a king's stead. In Scandinavia, it stopped being used in the Middle Ages, whereas, in Britain, it became synonymous with the continental count. In Anglo-Saxon times the title was similar to that of Ealdorman.
Today, an earl is a member of the British peerage, and is below a Marquess, but above a Viscount. A British Earl is the same as a continental Count. Since there is no feminine form of Earl, the wife of an Earl has the rank of Countess (the continental equivalent).