When you use an external flash or strobes with your camera, you need a way to connect them so when you press the shutter button down, the flash knows to fire. (Continuous lights don't need to be connected to the camera). There are a variety of ways to do so.

Hot Shoes

Many cameras have a hot shoe into which you can slide and lock an external flash that's designed to work with the camera. The electrical connections to the camera's shutter release and autoexposure system are made automatically when the flash is locked in place. There are also adapters for the hot shoe that let you connect strobes or off-camera flash units.

PC Terminal

A PC (Prontor-Compur) terminal located on some camera bodies lets you use cables to connect a flash or strobe. The cable that attaches to the PC terminal is called a sync cord (for synchronization and pronounced "sink") or PC cord. When you take a picture, a signal is sent from the camera along the cable to fire the studio flash. A few cameras have what looks like a hot shoe but lacking electrical connections. These cold shoes just hold a flash in place and require a cable and a PC terminal for the electrical connection. If your camera lacks a PC terminal but has a hot shoe, you can use an adapter in the hot shoe to connect a sync cord.

One use of the PC terminal is to move the flash off camera. The flash built into the camera is very close to the lens and fires along the same axis. For more interesting lighting effects, and shadows that show texture and relief, you have to position the flash further from the lens axis using a sync cord. The flash can then be handheld or mounted on a bracket or tripod.

When connecting an external flash or strobes to the camera you should consider using a voltage regulator. These devices reduce the sync voltage, protecting your camera from voltage spikes that could damage it.

Wireless Remote Flash

If you have one or more external flash units, you can make them into mini strobes using remote flash triggers. One of these inexpensive devices (some flash units have them built in) make any flash into a slave unit by firing it when it senses a flash firing elsewhere. This allows you to get lighting effects you couldn't possibly get with a single unit. More advanced flash units accomplish the same goal using optical or radio signals. You mount a master flash or a transmitter on the camera's hot shoe and it transmits wireless signals to the slave units telling them what settings to use and when to fire. The master flash on the camera can be enabled or disabled. When disabled, it still transmits signals to the remote units. With expensive units, the output ratio of different slave units can be set to finely tune exposures. This is ideal for background or accent lighting when shooting in a studio setting. When using wireless remote flash, you can use a modeling light that illuminates the subject for a full second so you can preview flash effects such as shadows and highlights before taking a picture.