If you are asking the difference between the D5100 and the D3300, read the DPReviews. In general terms, the only differences between cameras is going to be features. As example, I don't believe the D3300 offers bracketing. While the D5100 offers the basic 3-frame bracketing. Now maybe bracketing doesn't mean anything to you. But that is an example of the differences between cameras. Now this comparison is rather complicated as the D300 is newer technology that the older 5100. This means that the sensor might be up to one-stop better in low light (the ability to use a higher ISO). However, I might think that this is offset by such a high megapixel count. High megapixels tend to reveal any flaws in a photographers technique, which might not be a good thing for casual use.
Beyond that, you must understand that the only thing a camera does, any camera, is record an exposure. A camera records light. Period. My light meter will tell me the value of the light falling on a subject. These parameters (ISO, Shutter speed and Aperture value) will be the SAME on any camera because that is the light. There isn't one camera that can record the light "better" than another camera. A camera cannot control the quality, direction, color, intensity or subsequent contrast of your light. It only records it. The camera lens will play a MUCH bigger role than the camera. As example, the 18-55mm kit lens allows for an aperture of f/5.6 at 55mm. The 50mm f/1.8 prime allows for over 8 times more light to be captured. But it doesn't zoom! Photography is always a compromise. This is some believe a mirrorless camera might be a better option: at least the lenses will be cheaper and the lens is more important than the camera.