Everything you need for the right flash choice
Types of Flash
Hotshoe Mounted
These type of flash sit on your camera's hotshoe and work either independently or with your camera's TTL system. Some models feature swivel and tilt heads, for bouncing light around.
RingFlash
For macro and fashion photography, a ring flash is ideal as it provides an even concentration of light around the lens, helping to eliminate shadows.
TwinFlash
Similar to ring flashes in their suitability for macro work, but these feature two flash units around the lens, which can be supplemented with additional flashes.
Hammerhead
Generally offering more power than hotshoe mounted models, these sit alongside the camera's body on brackets and provide a better, off-centred lighting angle.
Choosing A Flash
Many DSLRs have built-in flash units, which are adequate for general snaps, but have limitations. If you want to get creative with flash, or just fire out more power with better light, a separate unit is essential. Like lenses, though, you need to find a flash that's compatible with your DSLR camera - a Canon flash won't work with a Nikon camera, for example. There are also independent manufacturers who make versatile and powerful units in a variety of fits, while you may want something more specialised such as a hammerhead or ring flash. Modern units offer tremendous automation and much of the mathematics that used to be required when using flash is now redundant. However, manual control is still useful to have for unusual situations or creative purposes. Another advantage of a separate flash unit is that it will offer faster recyling times than on-camera units, which is useful if you are taking many photographs quickly. There is also battery power to consider; flash draws a lot of power, and this can seriously reduce the life of your DSLR camera battery, but a flashgun has its own batteries - usually a set of AAs. Finally, the danger of redeye is reduced because the light is further from the lens axis.
What to look for
When buying a flashgun, it is worth bearing in mind that you generally get what you pay for. Cheaper flashguns are a mainly automated affair, with a few preset settings that match with the aperture that you are using. These are fine for the beginner or occasional user. More expensive models benefit from better build quality, greater coverage and manual control.