Most electrical socket boxes have three wires coming in: a black "live" wire, a white neutral wire and a copper or green ground wire. A red wire is usually a second live wire, and there are a few reasons why it could be there.
The most common reason for a red wire is to allow one of the two outlets to be operated by a switch in the room. In that case, the black wire supplies power to one outlet, which is always "on," and the red supplies power to the other outlet when the switch is in the "on" position.
Standard household wiring carries 120 volts of electricity, but some appliances—such as electric dryers and ranges—require 240 volts. A box wired for 240 volts would have two live wires—a black and a red.
It's also possible that whoever wired the outlet used four-wire cable (black, white, red and ground) rather than the standard three-wire cable (black, white, ground) because that's all he had available. In that case, the red wire serves no function.
A voltage tester can tell you whether the red wire is live. Shut off the power to the outlet, separate the wires, turn the power back on and then test the red wire. Make sure any switch that might control the red wire is set to on. Shut off the power again immediately after testing.
If the red wire isn't live, or if it is live but you don't need it, simply cap it off with a wire nut.
Always turn off the power before handling wiring, and keep children away from an exposed outlet box.