Compact cameras, entry level and semi-pro DSLRs and now even many smartphones have built-in flash lights, while professional full frame DSLRs usually don’t have one. Flash units can be used with almost any kind of DSLR, but recently there are even external flash units available for mirrorless cameras.
There are many different flash units, and you can choose the one you need depending on the photo you want to shoot, the approach you want to have with your subject and of course the money you can spend. In general they will offer better quality than a normal pop-up flash mainly for three reasons: they have more options to set, they are more powerful and they don’t use the camera battery to recharge.
An external flash unit gives you the possibility to choose between different using modes:
Other features are the possibility to choose the vertical and horizontal angle of the flash, the built-in diffusers and the presence of a smaller second light besides the main one, useful to enlight the shadows when the main light is not directed to the subject.
If you are planning to purchase a flash unit, the features you should take into consideration should be the ones described together with the recharge time of the unit which is the lapse of time it needs to be ready after an emission for another usage.