Equipment requirements

In USPSA you can use almost any center fire pistol you already own as long as it is safe, reliable and is of at least 9mm or greater in caliber.   Because some stages require you to shoot 32 rounds (the maximum number of rounds is 32), you will also need to be able to reload your gun during that competitive stage.  This normally requires you to have at least 3 extra magazines for your semi automatic (or moon clips for your revolver) and enough mag pouches on your belt to carry the ammunition.  No matter what kind of pistol you bring, you will only compete with people who have similar equipment or divisions.  

So, tell me about “Divisions”
Divisions are set up so that only people with similar equipment compete against one another. You’ve seen the fancy Star Wars looking “Open” Division guns on TV. You know the one that costs more than your first car and has a rate of fire on par with a Thompson submachine gun? Unless you’re in “Open” Division, you don’t compete against those guys. The Divisions are these:

Single Stack: 1911 style pistols only, with restricted holster types and limited modifications to the gun.
Production: your basic, ‘out of the box’ guns with restricted holster types and limited magazine capacity.
Limited: Fancy holsters are okay, and some modifications to the gun are allowed. You can’t use a compensator or electronic/optic sights.
Limited-10: Like Limited, but magazine capacity is limited to 10 rounds.
Revolver: Because some people don't want to have to pick up their brass
Open: Essentially unlimited modifications to the gun, optic sights, compensators, and uber-high capacity magazines. Anything goes in Open, and you are only limited by your budget.

So, tell me about the scoring system
Scoring is actually quite simple, The stage has a specific number of targets that you must engage or shoot.  At the start of a buzzer, you shoot the targets as fast and accurately as you can.  When you finish, the accuracy of your shots is divided by the time that it took you to calculate a 'hit factor'.  This hit factor is compared to other shooters nationwide to determine your overall classification

So, tell me about “Classifications”
Classifications are like Divisions, except Classification refers to the shooter’s skill level instead of his or her equipment. This is so only shooters of like ability compete with each other—if you’re just starting out you will not compete against a Grandmaster.
Once you become a member of USPSA, they begin tracking your scores on special stages called “Classifiers”. They are blueprinted to the inch and are shot the same way by every USPSA club. Your scores on a given classifier are then compared to the best score on that classifier and expressed as a percentage of that best score. When you get 4 on file, they take an average of your percentages and classify you with that. It looks like this:
U class—Not enough classifiers on file
D Class—0-39.9%
C Class—40-59.9%
B Class—60-74.9%
A Class—75-89.9%
You can classify in every Division separately. In other words, you can be a B Class Production shooter and a D Class revolver shooter. The exception is Single Stack, which for some reason just uses your highest classification.