A baronet (traditional abbreviation Bart, modern abbreviation Bt) is the holder of a title of honor (a baronetcy) created by King James I in 1611 to raise funds. It is an hereditary title, but is not a peerage title.
Baronets use the title "Sir" before their name, just as other knights do. The difference between baronetcies and all other knighthoods is that a baronetcy is effectively an hereditary knighthood, whereas all other knighthoods apply to an individual only. The eldest son of a baronet who is born in wedlock is also entitled to accede to the baronetcy upon the death of the current baronet, and to use the title "Sir".
Baronetcies, with a few exceptions, can only be inherited by, or inherited through, males.
These rights, however, were questioned by kings beginning with George IV, who held that the past sovereigns who created baronetcies could not bind future sovereigns. It is now rare for new baronetcies to be created, but one notable recent example is that of Sir Denis Thatcher, the husband of former Prime Minister Margaret Thatcher. Upon his death in 2003, their eldest son became the 2nd Baronet.
Baronet is not a peerage title and does not disqualify the holder from standing for election to the British House of Commons. However since 1999 neither do hereditary peerages, so the distinction has become historical. A full list of British Baronets can be found in Burke's Peerage and Baronetage.
Some notable baronets: