Here you can see three different exposures of the same shot. The middle one was normal, the left one was +1 overexposed and the one on the right is +1 underexposed.
Changing exposure here is called exposure compensation – on a DSLR there is usually a button with +/- on it – look for it on your camera on the body or on the LCD screen. Most cameras have this somewhere – look in your manual if you can’t find it. So you can let more or less light into the camera according to taste. Let’s look at the options visually
The camera has done a good job with the original exposure considering the sky is light.
Because we can change the light ourselves we can look at our result in the LCD playback and decide if we want to let in more of less light (most cameras allow thirds of a stop (here +1 means one whole stop). So you see you can vary the light in photography in a small or big way.
The overexposed image looks a bit washed out especially in the sky – note the highlights on the left side of the Brighton Pavilion dome – the detail is lost. Look at the difference in the skies: definitely we would want to choose one of the other two or a choice in between them (by taking another photo at minus one third of a stop).
We don’t mind the -1 photo on the right at all – this looks even better when seen fulllsize – perhaps if you were making a record of statues where you wanted as much detail as possible in the darks you might choose the middle picture (you see more detail in the full size picture).
So you choices depend a lot on what you want it for or what you find attractive in your photography – with exposure compensation you get to make the choices.