In online gaming, a tag is a short sequence of letters, numbers, and/or odd ASCII characters added to a player's name. It usually represents membership in a guild, clan, or group but it sometimes it is used to indicate other things.
Many online games do not provide a mechanism to indicate membership, so in these cases the player adds the tag as an actual part of his/her screen name. Most Roleplayers frown on this, however, because it makes for some ugly, nonsensical names. Also, if the group the player belongs to is dissolved, he/she may be stuck with the tag in the name.
Here are some examples of tags in names:
For those of you who are considering adding a tag to your screen name, please reconsider. If you must do it, please do not use l33t sp34k. That just makes it a pain to read.