Copyright is the legal right given to the originator of content so that they are protected from others using their content. Things such as books and movies are copyrighted to protect the original creator of them. If we are reading a book and Miss Tori wants to change a few words and then sell the book to other teachers she would be in trouble because of copyright. You are not allowed to use someone's work and then reproduce and sell it for any profit. The work does not belong to us and the work is protected under copyright law.
Fair use is what allows us to use the material and quote things in certain circumstances. If Miss. Tori found this really cool video for math about adding 2 digit numbers. that she wanted to show all of her class. She would be able to use it because of fair use. If we are wanting to show something to help educate or inform others we could as long as we are not trying to pass it off as our own work or trying to make a monetized profit from it.