A standard over in cricket contains six legal balls. One bowler delivers all six balls from one end of the pitch, after which a different bowler delivers the next over from the opposite end.
The word 'legal' matters because wides and no-balls do not count toward the six-ball total. If a bowler sends down an illegal delivery, the batting side receives an extra and another ball must be bowled. An over can therefore require more than six actual deliveries even though it always contains six legal balls.
Older forms of cricket used different over lengths, including four-ball and eight-ball overs. Six became the standard in modern international cricket and is used in Test matches, One Day Internationals and Twenty20 cricket. After an over ends, the batters usually change ends because of the completed six deliveries, while the fielding positions and bowler rotate according to the laws and match conditions.