Taken from wikipedia’s definition of the game:
The rules of tag are very flexible. Rules such as the following can be either decided upon before the game, or added as the game progresses to make play more fair.
At the beginning of the game, one player is designated “it”. After “it” is chosen, the other players scatter. “It” must chase them down and tag them, usually by tapping them somewhere on the body. A tagged player becomes “it”, and the former “it” joins the others in trying to avoid being tagged. This process repeats until the game ends.
In a typical game of tag, no score is kept, nor is a winner selected. Those who can avoid being tagged or who can stay “it” for the least amount of time are generally regarded as the best players. There is usually no time limit; the end of the game is chosen arbitrarily, perhaps when the players tire of the game, when recess ends, or when players get called home for dinner.
An anomalous property of tag is that although being “it” gives a player the most influence upon the game and thus could be considered the best role to play, the position is stigmatized and avoided. While most agree that the temporary stigma associated with being “it” is harmless, some have criticized tag because, they allege, a player who is often pursued to be made “it” or who is physically slow can be singled out and embarrassed. Because of this, tag has been banned in some US schools. In some variations, if the number of people exceeds five, then you may not quit until you are tagged first.