Kibitzing is giving commentary to an ongoing game. The person giving the kibitz is a kibitzer.
While in real life kibitzing on a serious game within earshot of the players is considered extremely rude, online kibitzing is far more lenient.
Many servers have a chat box to the right of the board. Using KGS as an example, the chat box is shared by the players and the spectators. Comments by the spectators are visible only to the spectators, while comments by the players are visible to everyone. This way, the spectators can discuss the ongoing game without disrupting the players.
It is wise to be polite to the players commenting as a spectator. While the comments are invisible during the game, they become visible to the players as soon as the game is over. Thus, your comment, "White is a really stupid player" on move three of the opening may be invisible at first, but the person playing white will see this after the game and most likely be offended.
- The art of kibitz (humour)