Mechanics of dismissal
Laws 27 to 29 discuss the main mechanics of how a batsman may be dismissed.
Law 27: Appeals. If the fielders believe a batsman is out, they may ask the umpire "How's That?", commonly shouted emphatically with arms raised, before the next ball is bowled. The umpire then decides whether the batsman is out. Strictly speaking, the fielding side must appeal for all dismissals, including obvious ones such as bowled. However, a batsman who is obviously out will normally leave the pitch without waiting for an appeal or a decision from the umpire.
Law 28: The wicket is down. Several methods of being out occur when the wicket is put down. This means that the wicket is hit by the ball, or the batsman, or the hand in which a fielder is holding the ball, and at least one bail is removed.
Law 29: Batsman out of his ground. The batsmen can be run out or stumped if they are out of their ground. A batsman is in his ground if any part of him or his bat is on the ground behind the popping crease. If both batsman are in the middle of the pitch when a wicket is put down, the batsman closer to that end is out.