A tag out is when a fielder, while holding the ball in his hand or glove, touches a base runner who is not safe on a base. The result is an out, and the base runner is removed from the field and an out is recorded for the defense. If the base runner steps off the bag at any point, he is unsafe and can be tagged out. However, it is not considered a tag out, if the fielder drops the ball while making the tag. Instant replay is sometimes used by umpires to review a tag out to make sure the base runner wasn't safe on base. Base runners will be called out if they try to avoid being tagged out by running outside of the base path or running lane.
Base runners must touch the bases in order of first base, second base, third base, and home plate in order to score a run. If at any point the base runner fails to touch a base, he can be tagged out by a fielder even if he is touching a base. For example, if a base runner fails to touch second base before reaching third base, but is safe on third base, the fielder can still tag him out.
Anytime a base runner is advancing the bases, he can be tagged out by a fielder who has possession of the ball. If a fielder has the ball and is near a base runner who is in the process of running between two bases, the fielder can simply touch the base runner with his glove, and the runner is out. This is called a tag out.