From BR Bullpen
An ejection occurs when one of the umpires removes a player, coach or manager from the game. This is usually the result of unsportsmanlike conduct (i.e. arguing too vehemently about an umpiring decision), but it can also be the result of specific action such as a pitcher deliberately throwing at a batter (see: beanball) or doctoring the baseball.
A person who is ejected must leave the field quickly and entirely (the field in this case includes the dugout and parts of the stands from which he could influence the ballgame) and not return until the game is concluded. Failure to comply can lead to further sanctions such as a suspension and/or a forfeit. An ejected player must be replaced by a substitute, while an ejected manager designates someone else, usually a member of the coaching staff, to take over his duties. The umpire who ejects someone must file a report with the League explaining the reasons for the decision, and could face sanctions himself if the ejection is deemed unjustified by the League President or another similar authority figure.
Non-uniformed personnel can also be ejected. This includes mascots, bat boys, guests of the team in the dugout or any other person who, in the judgment of the umpire, is disrupting the flow of the game.