Baseball features fairly straightforward rules for substitution. The roster is big enough at the high school, college and professional levels to allow for starter to spend the rest of the game on the bench without re-entering when replaced by a substitute. This is unlike basketball, hockey and football, with free substitution at all positions.
The most frequent substitutes in organized baseball are pitchers. Complete games are a rarity in modern-day baseball, and managers come out to change pitchers with great frequency. When one pitcher comes out of the game and a new pitcher comes in, the old pitcher is done and cannot re-enter the game at the elite levels of the game. Still, a cagey manager can make one move to allow the pitcher to come back to the mound after a substitution. If the manager sends the first pitcher to a different position, such as left field, he can have the relief pitcher pitch to one or two batters and then bring the pitcher back in from left field, since he did not leave the game.
When a pinch hitter comes into the game for any batter, he replaces that individual in the lineup. The player who has been pinch hit for is done for the game. A pinch hitter may not even get his at bat, if he gets pulled from the game and his eligibility is used up. For example, the manager could send up a left-handed batting pinch hitter to bat for a player. The opponent may then bring in a left-handed pitcher to pitch to that pinch hitter. This is not an optimal scenario for the hitter. The manager of the batting team may then send in a right-handed pinch hitter to bat, even though the previous pinch hitter has not even had one swing. Once a player is announced into the game and pulled, he cannot be put into the game at a later point.
Pinch runners are brought into the game when a slower player has reached base and the manager wants to replace him with a faster player. This is usually done in the late innings of a close game. The pinch runner becomes the substitute for the player he has replaced. He may take that player's spot when the team goes back into the field, but if he is not in the field, he is done for the game.
All-Star Game Exception
There is one exception to the substitution rule. In the Major League Baseball All-Star baseball game, both sides usually have two or three catchers on the roster. If the last catcher to be used gets hurt or has to come out of the game, one of the other two catchers can go back into the game. The reason for this is that managers of the All-Star teams usually try to get as many players into the game as possible, and the feeling was that the manager should not be penalized for doing that if the last catcher used was to get hurt in the game.