Copyrighting was created so that people cannot simply copy someone else's work and use it as their own. To use someone else's work (music, art, pictures, etc.), you must get permission. Copyright certificates can be obtained as protection against this. Because there is a copyright law, it is important to understand the consequences of stealing what is not yours.
Fair Use tells users what is allowed to be used of other people's work without permission. It creates a barrier between what is allowed and what is not. Fair Use has few limits and can be interpreted in various ways.
Here are some articles I found for further explanations of each.