Studs are the vertical beams that frame your house; they're normally hidden behind your walls. When you need to hang something heavy like a flat-screen TV, you have to make sure to drill into a stud so that it has enough support. Of course, if the studs are hidden within your walls, it might be a bit tricky to find them.
The easiest way to find a stud is to use an electronic stud finder. As you pass the device along your wall, it detects density differences. When an electronic stud finder hits a more dense area, it notifies you by beeping or lighting up. There are also magnetic stud finders that detect any screws or nails that were used to attach the wall to the stud. But if you don't have an electronic or magnetic stud finder, you can still figure out where studs are pretty easily.
One option is to scan the wall for electrical outlets, light switches or where a nail runs through the molding. These are all usually attached to a stud, so if you find one stud that way, you can measure off 16 or 24 inches (40.6 or 61 centimeters) in either direction to find the next stud, since that's the standard spacing for studs. If you remove an outlet cover, you may even be able to see the stud it's attached to. Other low-tech location methods include holding a lamp about an arm's length away from the wall to scan for depressions or dimples that signify where the wall is attached to the stud or knocking gently on the wall to listen for high sounds that suggest a stud. Where there's no stud, you'll hear a more hollow sound. If you make a mistake and drill in the wrong place, you can stick a bent piece of stiff wire in the hole and feel around to locate a nearby stud.