One of the most important settings on your camera is one you may not even know you had. Unless you're shooting in full manual all the time (check you out Mr Fancy-Pants!) you have probably had shots ruined that would have been great, if only you had known about Exposure Compensation.
Let's start with your camera, It's your loyal servant, your faithful friend. Unfortunately in spite of all the electronics inside, your beloved camera is very stupid. It's not really your poor camera's fault though, it's just doing what it was designed to do by the smart men in white coats over at Nikon/Canon/Fuji etc HQ.
You see, if you strip away all the bells and whistles, your camera is basically just a light meter in a sealed box. Light meters are great, photography wouldn't be anywhere without them, the problem is that (without wanting to anthropomorphise them too much) they get confused and panic very easily.
Let's say for example that you're playing in the snow with your dog, who is a dark-ish brown colour. To your eye the scene looks great so you pull out your camera and take a photo to show your parents on Facebook later, you get home and, damnit! The snow is an awful grey colour and the dog might as well be a black blob in the middle, you can't even tell what colour he is, it's just awful.
Here's what happened in that split second: Your camera's light meter analysed the scene and noticed that the snow was very bright and tried to compensate by lowering the exposure in response, rendering the snow as an average value instead of a bright one. This bumping of the overall exposure meant that everything else in frame gots darker too, so not only is the snow that horrible grey, your dog is practically in silhouette.
Don't blame the camera, in addition to being an inanimate object it is also not psychic, it didn't know what you intended and just went along with what it would normally do. You have a tool to directly tell the camera what you intend without having to go into full manual mode and get lost in twiddling dials.
As long as your camera is sufficiently advanced enough you should have either a button or dial with this symbol on it:
This is the Exposure Compensation symbol, a square divided in half diagonally, one half black with a white plus and the other black with a white plus. Press the button and turn the related control dial and you are able to communicate exactly what adjustments you want the camera to make when it does its calculations. As an aside, if you're shooting your camera's own RAW file you can easily tweak this later in Adobe Camera Raw or Lightroom (within reasonable limits, usually +/- 3eV at the cost of some increased noise in the shadows).
Rerun the snowy scenario with this in mind: You notice that the snow on the ground is reflecting a lot of light so you tweak your exposure compensation up by one whole stop (+1eV), communicating to the camera that yes; it's a bright scene, please don't panic; and yes you would like the photo to look bright too. You take the photo. Bingo! The white snow is a nice bright shade and Fido looks the same colour as when you see him, and his coat is full of detail. A photo worthy of gracing your Facebook!
This works the other way as well. If you're trying to take a photo of something dark, or at night, your camera will panic and try and brighten everything. Lights in the photo turn into glaring white blobs while everything else is usually noisy or blurred because your camera just can't shoot with a high enough shutter speed to beat the camera wobble in the dark. It's trying to exposure your night scene as though it were daytime, absent a tripod, it's not going to succeed.
Relax. Just adjust your exposure compensation downward. The photo will be darker as a result (it's night-time, it's dark, that's ok) but at least your camera isn't straining to do the impossible.
Of course, you could turn the flash on but that brings its own host of issues (more on that in another blog post).
This isn't just for the night-time though, anything dark will have the same effect on your camera. Try and take a photo of a black jumper or a PS4 and you'll run into the same problem, dial that compensation down.