The Comprehensive Rules covers this:
104.4. There are several ways for the game to be a draw.
104.4a If all the players remaining in a game lose simultaneously, the game is a draw.
104.4b If a game that's not using the limited range of influence option (including a two-player game) somehow enters a "loop" of mandatory actions, repeating a sequence of events with no way to stop, the game is a draw. Loops that contain an optional action don't result in a draw.
104.4c An effect may state that the game is a draw.
104.4d In a multiplayer game between teams, the game is a draw if all remaining teams lose simultaneously.
104.4e In a multiplayer game using the limited range of influence option, the effect of a spell or ability that states that the game is a draw causes the game to be a draw for that spell or ability's controller and all players within his or her range of influence. Only those players leave the game; the game continues for all other players.
104.4f In a multiplayer game using the limited range of influence option, if the game somehow enters a "loop" of mandatory actions, repeating a sequence of events with no way to stop, the game is a draw for each player who controls an object that's involved in that loop, as well as for each player within the range of influence of any of those players. Only those players leave the game; the game continues for all other players.
104.4g In a multiplayer game between teams, the game is a draw for a team if the game is a draw for all remaining players on that team.
104.4h In the Emperor variant, the game is a draw for a team if the game is a draw for its emperor. (See rule 809.5.)
104.4i In a tournament, all players in the game may agree to an intentional draw. See rule 100.6.
October 22, 2012 2:12 p.m.
To address rckclimber777's statement, we can view 2.5 from the Magic Tournament Rules:
If the match time limit is reached before a winner is determined, the player whose turn it is finishes his or her turn and five additional turns are played in total. This usually means that one player takes three turns and the other two, but a player taking additional turns may affect this. Team tournaments featuring multiple players playing together (such as Two-Headed Giant) use three turns instead of five.
If the game is incomplete at the end of additional turns, the game is considered a draw.
October 22, 2012 2:14 p.m.
A way of a draw in standard now is 3 Oblivion Ring s on the field with no other nonland permanents on the field. It doesn't use the word "may" like Detention Sphere , thus making 3 Oblivion Ring s a mandatory loop.
rckclimber777 says... #1
Draws typically happen when time runs out in a match. Matches have a time limit, if you don't complete the match in the proper time, then whatever game you're on ends in a draw. If you're on game 3 then it ends in a match draw as well. Another thing that can happen is that if everyone is at 3 life or less and then someone casts a spell such as Slagstorm which then drops everyone to 0 or less life. In other words, if an effect would eliminate all players currently in the game at the same time. All players who have 0 or less life are eliminated based on state based effects. Thus if everyone gets to that point at the same time then they are all eliminated at the same time and that results in a draw.
October 22, 2012 2:11 p.m.