IDPA equipment rules are very much like USPSA's rules for Single Stack and Production; not a lot of changes to what most would consider "stock" guns.
Course of Fire rules for IDPA emphasize that targets are "threats", and that you treat them as if they're shooting back, by engaging from positions of simulated cover; walls, trees, vehicles, etc. Since the scenarios are supposed to represent "realistic" (somewhat plausible) defensive situations, the number of targets and number of threat targets is low. In my local area, typical USPSA matches are six or seven stages, and 150-200 rounds. Typical outdoor IDPA matches are the same number of stages, but only about 100 rounds. The two sports are much more similar than they are different.