No-ball

A delivery which is deemed illegitimate. The penalty is one run to the batting side plus an extra delivery.

There are a number of reasons for an umpire to call a no-ball.
Front foot (overstepping). This is the common one. Some part of the bowler's front foot must be behind - not on, behind - the line. In limited overs cricket, this also incurs a free hit where the batsman can not be out (unless run out).
Back foot. The bowler's back foot must not intersect the return crease.
Waist-high beamer. A delivery which does not pitch and is above waist height on the batsman.
Intimidatory bowling. If the umpire feels the bowling is too aggressive, such as the infamous Bodyline series or the West Indians in the 1980s. Too much short-pitched bowling aimed at the body is what generally falls under this bracket.

No-balls can also be bowled to order and then revealed in a tawdry series of articles in a low-quality British tabloid newspaper.