In North America standard "mains" power was two-wire (hot and neutral) in many places. Within the last 50 years, the standard has gone to 3 wires, hot, neutral, and ground. The reason was mainly for safety. The replies before correctly state that any current flows between the hot, through the load, to the neutral, and then the reverse, mains being AC power. Since many machines include electric motors, to prevent shock from these machines the machine frames are connected to the ground wire in the mains circuit. If the motor winding shorts to the frame, for example, theoretically any current will flow through the ground wire and not through any person who happens to be touching it. You'll probably notice that most power tool bodies today are made from plastic and not metal as in the past. In addition to being more economical, the plastic bodies insulate the user from the power line. Whereas any device that has a metal housing is required to have a 3-wire grounded plug, plastic-housed devices are not, and commonly use a 2-wire plug. So if the power circuit and load are functioning properly, there should be no current flow through the ground wire.