Spot welding
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Spot welding is a type of resistance welding used to weld thin pieces of ferrous metal. It uses two large electrodes which are placed on either side of the surface to be welded, and passes a large electrical current through them that heats up the metal in-between. The result is a small "spot" that is quickly heated to the melting point, forming a small dot of welded metal. Applying the current for too long can burn a hole right through the material.
Spot welding is typically used when welding steel sheet metal. Thicker stock is difficult to heat up from a single spot, as the heat can flow into the surrounding metal too easily. You can easily identify spot welding on many sheet metal goods, such as metal pails. Aluminum spot welding is much more expensive as the equipment used must hold the metal at precise pressures at precise currents for precise lengths of time.
Perhaps the most common application of spot welding is in the automobile industry, where it is used almost universally to weld the sheet metal forming a car. Due to changes in the resistance of the metal as it starts to liquefy, the process can be monitored in real-time to ensure a perfect weld every time. This allows the spot welders to be completely automated, and many of the industrial robots found on assembly lines are spot welders (the other major use for robots being painting). One place where you might see a spot welder is in the orthodontist's clinic, where small scale spot welding equipment is used when resizing metal "molar bands" used in orthodontics.