Hi,
Maurice is very dead on with this one. They are both games and both have unique rules.
IDPA stresses scenarios that are more on line with "defensive situations". The rules require that the shooter shall not use a "race holster" and shall only carry a total of 3 mags. Courses of fire are not to exceed 18 rounds (correct me if I'm wrong here IDPA gurus). All draws are from concealment or, if you are serving Military or Police, then using ALL duty gear (i.e. armour, duty rig with retention holster and while wearing batons, OC, handcuffs, etc).
All mag changes are to be done from cover, use of cover is mandatory, targets are to engaged in tactical sequence (guns before knives or unarmed assailants, etc). Shooting on the move is sometimes dictated in the course of fire. Scoring is quick and dirty because points can be figured out immediately from hits on target added to time (C zone = +1, D zone = +2, mikes = plus five and there is also failure to neutralize i.e. no hits in a scoring area that would normally cause incapacitation and a mike in a defensive scenario).
A problem I have noticed is the debate on what is proper tactical sequence or what to do if you inadvertently drop a mag or exactly what tactics are. A course designer would have to be a tactical god to figure out all of these things and even then two highly skilled tactical people may disagree on what is right and wrong. I also don't like the idea of "failure to do right" penalties... it leaves too much room for an RO to screw over a competitor (yup I've seen it happen and it happened to me). I've seen guys fumble to pick up a dropped mag so that they wouldn't incur a 3 second procedural or a failure to do right..if it was a tactical situation then you'd have a bullet in the top of your head while screwing around trying to pick something up.
IDPA is fun and anyone who thinks that IDPA prepares them for a lethal confrontation is on crack or maybe should consider using crack 'cause then getting beat up or shot might not hurt as much and you'll be all messed up for court and have a great sympathy ploy at sentencing time!!!!
I shoot IDPA because it's fun, it isn't as politically correct as IPSC (as if any form of shooting sport is politically correct!?!

), I can use what I wear to train and to go operational and the shooters see it as FUN and not the end-all-to-be-all. (I know IPSC guys who think that EVERYONE knows who they are because they shoot IPSC... yeah, right!

) Also, IDPA shooters that I've been with don't take the sport too seriously, I haven't seen anyone argue with an RO yet, there aren't any shooters in their "ZONE" and people are actually polite!
I love IPSC but I don't like some competitors. I've learned more from IPSC than from IDPA (maybe it was timing but I don't think so). Both are fun and you should explore both. It's shooting that counts regarless of the discipline.
Get out and shoot..it's fun, wholesome and rewarding.