Lighting creatively is one of the biggest challenges beginning and amateur photographers have to face. First of all, there is the taboo of using flash that has been spread by hipsters and artsy types who only work with natural light because it's the purest. The reality of that is they just don't know how to use anything else.
With that said, let's start educating you about off camera flashes. They're also known as external flashes, speed lights or flash guns. They're basically flash units that you attach to your camera's hot shoe.
They have numerous advantages both on and off camera. Using them mounted on your hot shoe will give you a lot more power than you’re built in flash. Actually I recommend using the built in flash only in case of emergency. Other than the extra power, which is very important, especially in dark interior spaces, there are some very cool benefits that come with external flashes. The pop-up flash that's found on most non pro cameras can only be pointed straight at your subject, whereas the head of an off camera flash can be tilted 180 degrees horizontally and 90 degrees vertically. How does that translate in the real world? Softer, much better looking light. There are several techniques for getting this kind of light, the most common one is called bouncing and it's used especially by event photographers and photojournalists. The way it works is by pointing the flash head towards a white ceiling, thus using it as a giant reflector.
This is just one example of how to use one of these babies.
Flashes are like lenses. They can either be manufactured by the same company that made your camera, or by a third party. The major manufacturers, Canon, Nikon, Sony, Pentax and Olympus all have a flash lineup for at least one system. There are obvious perks to staying "within the brand". Let's say you own a Nikon (I’m just using it as an example, it could be any brand listed above) camera. Buying a Nikon flash, a Speedlite in this case, will give you some pretty solid features like TTL ( metering through the lens) and wireless triggering just to name a few.
The same goes for the other brands as well because their external flashes are designed for their cameras. Usually the build quality is also top notch. Buying within the brand will set you back financially but at least you know what you're paying for.
The alternative are third party flashes that either replicate the original models or have designs of their own. They lack most of the advanced technologies and can often be used only manually. The build quality also leaves to be desired in most cases, but there have been exceptions. Their biggest advantage is that they're dirt cheap and if you drop one, or two for that matter, during a photo shoot you won't feel the pain in your wallet.
When deciding what flash to buy, you should also consider what you need to use it for. As I've mentioned before, these flashes can be triggered off camera. The big boys like Canon and Nikon can be fired using internal systems that are found inside the cameras and flashes. For example you can use five independent flashes at a shoot and fine tune the power of each of them by pushing a few buttons on your camera. Third party flashes lack such features, but nevertheless they can still be used off camera with the help of triggers. These are sold in kits and they also come in different price ranges.
This is basically what you need to know before going shopping. As a final piece of advice, don't buy a flash before testing it on your camera. It's the only way to get a clear idea of how it works. After you buy it, be kind enough to yourself to read the instructions. Those things can be a lot more sophisticated than you would think. Happy shopping & shooting!