Crimp tools, or crimping pliers, are a special variation of pliers attach connectors to the end of wires. They crush, or crimp, the connector around the wire, holding it in place by friction. Automotive crimp tools can crimp three sizes of connectors, often color coded in red, blue and yellow for small, medium and large gauge wire. The connectors themselves also color coded in this manner, and are available at many auto parts stores.
Crimping requires a fair amount of force, and cheaper crimp tools are little more than pliers with recesses for different sizes of connectors, and require you to figure out how much force that is on your own. Nicer crimp tools feature a ratcheting action, and will not release until they have been squeezed tightly enough to ensure that the connector is firmly crimped onto the wire. In a pinch, a pair of pliers can be used as a poor-mans crimp tool.