Baronet is a hereditary title passed down through the male line. Like knights, baronets are addressed as 'Sir', the difference being that knight is a non-hereditary title. Having said that, a baronetcy is sometimes described as a hereditary knighthood.
Unlike the other main British hereditary titles (in descending order of precedence: Duke, Marquess, Earl, Viscount, Baron), baronets are not members of The Peerage or of the nobility but are classed as commoners.
Baronets have fallen traditionally into the category known as the landed gentry, well-off but non-titled landowners.
Previously, the abbreviated suffix Bart was used after a baronet's name. Nowadays, this is further abbreviated to Bt.