AndyP
You can go to
www.uspsa.com to learn more about USPSA/IPSC. To be a little more specific on some differences:
The targets are the same outline, but have different scoring areas.
The scoring method is different, but it still measures accuracy and speed in combination.
Knock-down steel "popper" targets are often used.
Single stages can be longer than those in IDPA, sometimes up to as many as 30 rounds. They also tend to be more varied and interesting.
To reload you just drop the old mag and put in a new one. No "tactical reloads" or "reloads with retention."
Pre-ban high capacity magazines are used in some of the gun divisions, but aren't necessary for Limited 10 and Production.
You can carry as many mags on your belt as you want. The most common number is 4.
There are different divisions for guns, Open, Limited, Limited 10, Production, and Revolver. If you don't have hi-cap mags, your IDPA gun will either be Limited 10 or Production. If you shoot 9mm, it's best to be in Production from a competitive point of view, but Production has rather restrictive rules on equipment to limit it to stock DA first shot guns.
Except in Production, fancy hi-speed "race" holsters are allowed, but not necessary. There are no draws from concealment.
In my experience, IPSC shooters are, on average, more intense and more competitive. They are still friendly and helpful, however. The people in the local club make a big difference in one's experience. I started in IPSC and later got involved with IDPA and have no confusion shooting both. I met a guy in IDPA and invited him to our IPSC matches and he was immediately hooked.
Good luck, and enjoy!
AndyP