Did you know that there are several different types of flash you can use? They fall under two categories, internal and external, and they can enhance your photos by providing extra light, if they're used in the right way.
Most DSLR cameras come with built in "pop up" flash (with the exception of the most expensive professional cameras). This type of flash can provide an extra source of light for up to about 8 feet (approximately 2.5 metres), which can be useful when you don’t have any other light source on hand.
You’ll find that this flash will "pop up" most frequently when you have your camera set to Auto mode rather than manual mode, especially if you are shooting indoors or in low light. This is because the camera will detect when not enough light is hitting the sensor, and try to overcome this by turning on the internal flash. The issue is that the internal flash produces a harsh, direct light that can result in red-eye (because it reflects off the subject’s eyes), and usually blows out your image. They also drain the camera battery much faster.
You can turn off your internal flash by going to your camera Flash Settings. Note that, once you have done so, you are likely to get underexposed images if you are using Auto and don’t have sufficient available light. The vast majority of professional photographers prefer to turn off their pop up flash, opting to use an external flash unit instead.
Electronic flash units are also commonly referred to as speedlights. This is the generic term for flash units; Canon spells its flash units as “Speedlight”, with a capital S, while Nikon flash units are called “Speedlite”.
External flash units are sold separately from camera bodies and lenses, and come in many shapes, sizes, and forms. They are a more flexible and powerful source of supplemental lighting, and you can control the angle and position of the unit to best suit your subject. So if your subject is in close range, you can move the flash to an angle so the light will not bounced directly off them, which stop the photos from looking overly bright. It takes time to learn how to use a flash unit, and balance the output of light with your other exposure settings.
When starting out with flash photography, it's important to understand how light affects your photos. Why not have a read of these twoposts about understanding light before you start?