If you're enough of a foodie to be considering an induction cooktop, there's a chance you've acquired some glass, copper or aluminum cookware over the years. If so, brace yourself, because you're not going to be able to use any of it on your induction cooktop -- induction cooktops require ferromagnetic cookware to work.
While it's theoretically possible to make an induction cooktop that's compatible with other types of metal cookware, the industry isn't currently headed in that direction, primarily because the amount of energy required to do so would make the units inefficient. Remember, induction cooktops use the high resistance of iron to their advantage, converting a little current into a lot of heat -- highly conductive materials like copper or aluminum would require much higher frequencies to make their small resistances add up to much heat. With this in mind, some manufacturers have begun adding an iron plate to the bottom of their nonferrous cookware to allow them to work with induction cooktops.
The good news is that you probably already have magnetic cookware in your house. Your cast-iron skillet is a sure bet, and much of your stainless steel should work fine, too. If you aren't sure if your cookware will work, simply hold a magnet to the bottom of it. If it sticks, you're in business. Also, some induction cooktops have built-in detectors that will tell you if a pot or pan is appropriate.