A wire stripper is used in removing insulation from around a wire or cable. Sure, you can do this with a pair of scissors, wire cutters or even a utility knife, but using a tool specifically designed for this purpose makes life easier if you work with wires often.
Here are three wire strippers I use to remove the insulation from electrical wires before soldering them or installing them to a connector (screw, crimp or spring loaded).
On the left is a cheap wire strippers that often barely work. It consists of two pieces of stamped steel with a long channel cut in one side. That channel has a nut and bolt which can be moved up and down in order to limit the size of the notch cut into the business end. This system can work OK even for very thin wires if you can adjust the size right for your wire. Adjusting and re-adjusting after changing wire type is annoying.
On the center are self-adjusting wire strippers from Biltema. It is especially useful when I am dealing with wires that have different thicknesses. This works well with modt electrical wires used on mains power applications down to many electronics hookup wires. The downside is that it does not work with all cables and this does not work well with very thin wires.
On the right there is wire stripper and wire cutter combination tool. This works quite OK for many electronics circuit wires.