ETOPS stands for Extended-range Twin-engine Operational Performance Standards.
It allows twins to operate up to a certain number of flying minutes from a diversionary airport. The number of minutes is calculated with the flying speed on one engine.
The several classes comes in with the number of minutes. There is ETOPS 180, ETOPS 207 and ETOPS 240. (Is there a 120?)
So if an engine fails, the aircraft is tested and certified to keep going on one engine to a diversionary airport located within a certain number of minutes. This allows the route to go over desolate stretches of ocean or land and potentially saves time and fuel.
Hawaii is the most isolated place in the world in this respect btw
Critics of ETOPS say it is a numbers game, and that eventually both engines will fail on some flight and there will be a big hole in the ocean. Proponents (including myself, not that I count much) say that if both engines fail the problem would have taken out all the engines on a triplet or quad anyway. Besides which engines almost never fail anyway, especially not in cruise.
Triplets and quads operating at these ranges from diversionary airports have to follow LROPS, Long Range Operational Performance Standards.
ETOPS is sometimes referred to as: Engines Turn Or Passengers Swim
[Edited 2004-09-25 23:27:45]