The word Earl is from the Anglo-Saxon magnate known as an ealdorman who was a local ruler.
The original term is from "jarl"- a powerful Viking Noble. Many former Prime Ministers were made earls when they left office.
Until the 17th century an earl was invested by the Sovereign with the sword he wore at his waste - hence the term 'a belted earl'.
Some Scottish earldoms pass through the female line. The present Earl of Mar is a woman.
The eldest son of an earl always bears one of his father's (or mother's) secondary titles. The other sons are 'Hons.' Daughters are styled Lady.