British nobility

The British nobility is made up of the peerage of the United Kingdom and the landed gentry. The nobility of its four constituent home nations has played a major role in shaping the history of the country, although now they retain only the rights to stand for election to the House of Lords, dining rights there, position in the formal order of precedence, the right to certain titles, and the right to an audience (a private meeting) with the monarch. More than a third of British land is in the hands of aristocrats and traditional landed gentry.
British nobility.
The British nobility consists of members of the immediate families of peers who bear courtesy titles or honorifics. Members of the peerage carry the titles of duke, marquess, earl, viscount or baron. British peers are sometimes referred to generically as lords, although individual dukes are not so styled when addressed or by reference.
A Scottish feudal barony is an official title of nobility in the United Kingdom (but not a peerage), and a feudal baron is addressed as "The Baron of X". Scottish lairds' names include a description of their lands in the form of a territorial designation. In Scotland, a territorial designation implies the rank of "Esquire", thus this is not normally added after the name. Lairds are part of Scotland's landed gentry and—where armigerous (that is, entitled to heraldic arms)—minor nobility.
All modern British honours, including peerage dignities, are created directly by the Crown and take effect when letters patent are issued, affixed with the Great Seal of the Realm. The Sovereign is considered to be the fount of honour and, as "the fountain and source of all dignities cannot hold a dignity from himself", cannot hold a British peerage.
Landed gentry.
Descendants in the male line of peers and children of women who are peeresses in their own right, as well as baronets, knights, dames and certain other persons who bear no peerage titles, belong to the gentry, deemed members of the non-peerage nobility below whom they rank. The untitled nobility consists of all those who bear formally matriculated, or recorded, armorial bearings.
Other than their designation, such as Gentleman or Esquire, they enjoy only the privilege of a position in the formal orders of precedence in the United Kingdom. The largest portion of the British aristocracy has historically been the landed gentry, made up of baronets and the non-titled armigerous landowners whose families hailed from the medieval feudal class (referred to as gentlemen due to their income solely deriving from land ownership).
History.
Before the 20th century, peerages were generally hereditary and (with a few exceptions) descended in the male line. The eldest son of a duke, marquess or earl almost always uses one of his father's subsidiary titles as a courtesy title; for instance, the eldest son of the Earl of Snowdon is called Viscount Linley.
The modern peerage system is a vestige of the custom of English kings in the 12th and 13th centuries in summoning wealthy men (along with church officials and elected representatives for commoners) to form a Parliament. The economic system at the time was manorialism (or feudalism), and the privilege of being summoned to Parliament was related to the amount of land one controlled (a "barony"). In the late 14th century, this right (or "title") began to be granted by decree, and titles also became inherited with the rest of an estate under the system of primogeniture. Non-hereditary positions began to be created again in 1867 for Law Lords, and in 1958 generally.
In 1958, the Life Peerages Act 1958 enabled (non-hereditary) life peers to sit in the House of Lords, and from then on the creation of hereditary peerages rapidly became obsolete, almost ceasing after 1964. This is only a convention, and was not observed by prime minister Margaret Thatcher, who asked the Queen to create three hereditary peerages (two of them, to men who had no heirs). Until changes in the twentieth century, only a proportion of those holding Scottish and Irish peerages were entitled by that title to sit in the House of Lords; these were nominated by their peers.
Until constitutional reforms soon after Tony Blair came to power (the House of Lords Act 1999), possession of a title in the peerage (except Irish) entitled its holder to a seat in the House of Lords. Since then, only 92 hereditary peers are entitled to sit in the House of Lords, of which 90 are elected by the hereditary peers by ballot and replaced on death. The two exceptions are the Earl Marshal (a position held by the Dukes of Norfolk), who is responsible for certain ceremonial functions on state occasions, and the Lord Great Chamberlain (a position held in gross and one of a number of persons can hold it), who serves as the monarch's representative in Parliament and accompanies them on certain state occasions; both are automatically entitled to sit in the House. Typically, those due to inherit a peerage—or indeed have done so, in recent times—have been educated at one of the major public schools, such as Eton, Radley, or Harrow.
A member of the House of Lords cannot simultaneously be a member of the House of Commons. In 1960, Anthony Wedgwood Benn inherited his father's title as Viscount Stansgate. He fought and won the ensuing by-election, but was disqualified from taking his seat until the Peerage Act 1963 was passed enabling hereditary peers to renounce their titles. Titles, while often considered central to the upper class, are not always strictly so. Both Captain Mark Phillips and Vice-Admiral Sir Timothy Laurence, the respective first and second husbands of Princess Anne, do not hold peerages. Most members of the British upper class are untitled.
Irish and Gaelic nobility.
Outside the United Kingdom, the remaining Gaelic nobility of Ireland continue informally to use their provincial titles, few are recognised as royal extraction by the British Royal Family such as O'Donovan family. As Ireland was nominally under the overlordship of the English Crown for between the 12th and 16th centuries, the Gaelic system coexisted with the British system. A modern survivor of this coexistence is the Baron Inchiquin, still referred to in Ireland as the Prince of Thomond. The Prince of Thomond is one of three remaining claimants to the non-existent, since the 12th century, High Kingship of Ireland, the others being The O'Neill, MacCarthy Mor dynasty and the O'Conor Don
Chief of the Name was a clan designation which was effectively terminated in 1601 with the collapse of the Gaelic order, and which, through the policy of surrender and regrant, eliminated the role of a chief in a clan or sept structure. This does not mean there is no longer a Chief or a sept today. Contemporary individuals today designated or claiming a title of an Irish chief treat their title as hereditary, whereas chiefs in the Gaelic order were nominated and elected by a vote of their kinsmen. Modern "chiefs" of tribal septs descend from provincial and regional kings with pedigrees beginning in Late Antiquity, whereas Scottish chiefly lines arose well after the formation of the Kingdom of Scotland, (with the exception of the Clann Somhairle, or Clan Donald and Clan MacDougall, the two of royal origins). The related Irish Mór ("Great") is sometimes used by the dominant branches of the larger Irish dynasties to declare their status as the leading princes of the blood, e.g.MacCarthy Mor dynasty, lit. (The) Great Macarthy or 
Ó Néill Mór, lit. (The) Great O'Neill.
Following the Norman invasion of Ireland several Hiberno-Norman families adopted Gaelic customs, the most prominent being the De Burgh dynasty and FitzGerald dynasty; their use of Gaelic customs did not extend to their titles of nobility, as they continuously utilized titles granted under the authority of the English monarchy.