Definition: A torque wrench is a special wrench that indicates how much torque is being applied to whatever you are trying to wrench.
Manufacturers of bikes and components usually provide torque specs for screwing in anything that is threaded. This spec, usually in N-M, ft-lb, or in-lb, provides a safe mount of torque to be applied that will neither strip out the threads nor allow the threaded item to come loose.
Torque wrenches usually have a fitting or socket on the end to allow you to attach different wrench ends. You only need one torque wrench and a good set of sockets (with hex and other typical bike fittings) to work on most parts of your bike.
Examples: Oh, too bad about those stripped threads, maybe you should have used a torque wrench.