A peer is an equal, and fundamentally an equal in a legal context. From this developed peers of the realm in England and the pairs (originally twelve pairs) of France.
A peerage in Britain conferred a right of personal summons to Parliament. This was attached to the titles Baron (in Scotland before 1707, lord of Parliament), Viscount, Earl, Marquis (Marquess) and Duke. Since 1999, the hereditary peers no longer have a right to attend Parliament unless they are among the 97 elected representative hereditary peers, and only life peers can attend automatically.
In France most dukes and some counts ranked as pairs de France in the ancien régime and could sit in the Parlement of Paris. Six archbishops and bishops also ranked as pairs de France, whereas in England the archbishops, bishops (and before the Reformation certain abbots) rank as lords spiritual and sit in the House of Lords as such.
In France, a Chambre des Pairs was created in 1814 and peerages were given to a larger number of titled nobles. These were hereditary from 1815, although from 1817 an entailed supporting estate (majorat) was also required. From 1830, only life peerages were conferred and the Chambre des Pairs was abolished in 1848.
Also during the nineteenth century, this concept of peerage was extended to Sicily (1812) and Portugal (1826, with both hereditary and life peers, although the former were phased out from 1885).