To clarify it better: whenever the game enters a loop that CAN be stopped (for instance, with NBarden's example, you can replicate Pact of the Titan as many times as you want, since it costs 0, but you CAN choose to stop doing it), the player who can stop it chooses a number. The loop is considered to have occured that number of times, and then stops. So, there's no "infinite", per say.
On the other hand, suppose I play Oblivion Ring, and the only permanent in play it can target is a Mogg Fanatic, which gets sacrificed in response, so the RIng stays in play, bu has removed nothing. Then, a second Oblivion Ring is played, targetting the first one.
Now, you have a Ring in play, removing another Ring. If ANOTHER Ring is played, if there are no more permanents in play that the Rings can target, the game enters a NON-STOPPABLE infinite loop, since the Ring will remove the other Ring, making the first Ring come into play. Its ability will trigger, and you'll have to remove the third Ring, which in turn will return the SECOND Ring to play, triggering its ability, so on, so forth. If something like THIS happens, the game is ended instantly in a tie.