Ok, here's the deal.
States have their own laws, and some of them have passed laws banning high capacity magazines (like california), though most have not.
There was a federal law passed in 1994 that is usually called the Assault Weapons Ban or AWB, that, among other things, banned all NEW high capacity magazines (all magazine that could hold more than 10 rounds) with an exemption for law enforcement. So unless you are a police officer, it is illegal for you to own a new, or post-ban high capacity magazine, though you may still have the ones that were made before the ban went into effect.
This ban also banned many weapons based on their appearance (rather than their function) and was basically just a big hassle to gun owners everywhere and did absolutely nothing to prevent, or lower crime. This may be due to the fact that the weapons targeted by the ban (the larger 'assault' weapons group) are (and were) used in less than 1 percent of all gun crimes, but is probably mostly because the ban only made certain cosmetic features illegal (like a bayonet lug/flash suppressor).
Fortunately, this ban had a built in sunset clause, meaning that if it were not renewed by congress, it would expire in 10 years. The 10 years is up this september, and everything (including all high cap magazines) should be completely legal again.
So the short and skinny is that you just have to be carefull during the next couple of months if you purchase a pistol.