They are much more alike than different, but the differences can't be overlooked. If you watch videos of the two sports, it may not be clear about the differences, as not all matches, in either sport, are run strictly by the rules. The similarities are the targets themselves, that the scoring is a balance of accuracy, speed and power, and the general rules that govern safety and shooter behavior. The differences are mostly in how the targets are engaged. USPSA prides itself on its "freestyle" approach, in which the shooter is given a general overview of the shooting "problem", and the shooter is somewhat free to decide how to "solve" it (there are ten targets out there, engage them in any sequence). IDPA courses of fire are scenario-driven, with the problem and the solution generally laid-out in advance, and the shooter left to execute the solution better than anyone else (there are two badguys on the left side of the convenience store, and after you have engaged them, move to the right side and engage the other two, keeping the counter between you and the threats at all times). In USPSA, the targets are mere bullet fodder, to be ventilated as efficiently as possible. IDPA targets are "threats", to be treated as if they are shooting back. Poor course design blurs the distinction, and that can lead to a false impression of the differences. Equipment can be similar; any gun that is IDPA legal is also legal for USPSA competition, but it may not be as suitable as a gun that is tailored especially for USPSA.