Radio Slaves & Optical Slaves
A slave unit will detect a camera's flash and then it will fire off any other flash unit connected to it. Some accessory flashes have a slave unit built in, but you can buy separate units that fit on to other flashes. Slave units make it a bit easier to use lighting set ups that have numerous flash units.
There are two types of slave units, optical and remote. Optical slaves are generally less expensive, but aren't as reliable outdoors. The sensor in an optical slave will sense a flash and trigger off other flashes. Radio or remote models require a transmitter and receiver for each flash. When you take a photo the transmitter sends signals to the receivers which trigger the flashes. Slave units are wireless. Optical slaves can be bought from about $15 and up.