Copyright is a law that stops you and I from using other people's work without their permission. This includes pictures, music, videos, art, and even stories. The only way to get permission to use these things is by mentioning their name when you are sharing it to give them credit for the work.
Fair use allows us to use copyrighted work without the owner's permission. This is a very limited set of circumstances. An example of this would be using a song or a picture for a presentation at school.