Peerage of England
The ranks of the English peerage are, in descending order, Duke, Marquess, Earl, Viscount, and Baron. While most newer English peerages descend only in the male line, many of the older ones (particularly older baronies) can descend through females. Under English inheritance law all daughters are co-heirs, so many older English peerage titles have fallen into abeyance between various female co-heirs.
Peerages in the United Kingdom
Individuals who use the style "Lord" or "Lady" are not necessarily peers. Children of peers use special titles called courtesy titles. The heir apparent of a duke, a marquess, or an earl generally uses his father's highest lesser peerage dignity as his own. Hence, the Duke of Devonshire's son is called Marquess of Hartington. Such an heir apparent is called a "courtesy peer", but is a commoner until such time as he inherits (unless summoned by a writ in acceleration).