A truly Canadian analogy to the question would be OHL & NHL are to Hockey as IDPA & IPSC are to shooting sports.
And I am not trying say that one is better than the other, just that they are both governing bodies to a sport. Each governing body establishes a "rule book" for their sport, collects afilliation dues from clubs and/or membership fees and so on...
IDPA=International Defensive Pistol Association
IPSC=International Practical Shooting Confederation
(And I just noticed that while I was typing this the links were posted for you by Dragoon and maurice... damn you guys are quick.)
Albeit, the rule differences are quite subtantial between IDPA and IPSC they both are essentially run as a timed shooting events in which competitors engage paper and/or steel targets (some of which are moving targets) with accuracy and time playing a major role in the competitors score. Traditionally speaking IPSC and IDPA are a handgun sport (9mm or greater).
3-Gun differs only that in addition to the handgun component of the match, a shotgun, and a rifle is required in the match. (Hence the name 3-Gun)
3-Gun can be run under the auspices of the IPSC rule book, the IDPA rule book (I believe), or the roguish rules set forth by the director of a particular match.
Check out the links provided above and come on out and shoot. You can't go wrong with any of the above... they are all a pure adrenalin rush and a LOT OF FUN.
Cheers,