A hereditary title intermediate between peerage and knighthood. Bearers are called Sir So-and-so, and suffix 'Bart' or (sometimes said to be less preferable) 'Bt' after their name. They're a kind of knight (commoner), not a kind of baron (lord). A baronet, like a knight, is referred to as e.g. Sir Thomas Beecham or for short Sir Thomas (but never Sir Beecham), but his wife is Lady Beecham, without the forename.

Although anciently baronets had existed in both England and Scotland, there were none at the time James VI and I revived the title. It is a knighthood that is hereditary. The first batch of 200 baronets were rewarded for contributions of money or service, that is the titles were quite openly for sale.

The peculiarity of the Scottish titles was that they were granted as baronets of Nova Scotia, and a parcel of land in that settlement went with the title.

After the union of England and Scotland in 1707, new baronets were created as baronets of Great Britain, then after 1801 of the United Kingdom. There were also baronets of Ireland. After 1964 no baronets had been created until, Margaret Thatcher ceasing to be prime minister but declining the customary peerage, her husband was created a baronet, Sir Denis Thatcher. (She of course later did accept a peerage.)

In 1965 a female baronet was admitted for the first time, when the right to succeed was recognized for Dame Maureen Dunbar of Hempriggs, as a baronetess of Nova Scotia. (It may happen again, because her son has only daughters.) Normally baronetcies, like titles of nobility, can't pass to female because the remainder (the formula for eligibility of succession specified when it's created) in most cases speaks of heirs male.

The armorial peculiarity of English baronets, and those created after the Union, is that their shields bear the red hand of Ulster on a white canton.

Baronets of Nova Scotia have an orange-tawny ribbon, and the arms of Nova Scotia (blue saltire on white, reverse of those of Scotland) superimposed on theirs: not just the shield (as an inescutcheon), but the supporters as well, on top of the main shield. This is most unusual. Their motto is Fax Mentis Honestae Gloria... which either means glory is the torch of an honest mind... or a torch or a meteor or death or a wedding is the glory of an honest mind.

See also aneurin's write-up in Baronetage for more detail.