Today I am going to talk about the histogram which is I believe the defining thing for a "perfect exposure." The histogram is a graph of how much brightness vs darkness you have in an image. On the left we have black and on the right we have white, then bottom to top is how much you have in each. Some histograms like below have colors to go along with them and others just have a black and white graph.
The camera wants to make the images be in middle ground. Sometimes that doesn't leave us with an image that portrays the mood or feel of what we see at the moment. So we change it and take control away from the camera. If we are in the semi manual modes then we can use exposure compensation (it looks like a little +/- icon) to make the image our own. or go to full manual and have complete control over the exposure.
In manual if we want it a little darker or brighter we can just make an adjustment to the shutter, aperture, or iso to achieve the desired effect. each one can change the way the picture looks as previously discussed. Now go out and shoot. Enjoy it, experiment.