over of a cricket match

The bowler bowls the ball in sets of six deliveries (or "balls") and each set of six balls is called an over. This name came about because the umpire calls "Over!" when six balls have been bowled. At this point, another bowler is deployed at the other end, and the fielding side changes ends while the batsmen do not. A bowler cannot bowl two successive overs, although a bowler can bowl unchanged at the same end for several overs. The batsmen do not change ends and so the one who was non-striker is now the striker and vice versa. The umpires also change positions so that the one who was at square leg now stands behind the wicket at the non-striker's end and vice versa. In one day match a bowler can  bowl 10 overs. 4 overs can bowl in T20 match. But in a test match  a bowler can bowl many overs if he had a good fitness.