The 10 Myths of Copyrights
Brad Templeton has a great article on the ten myths of copyrights. As an artist I found this to be at the least entertaining.
“If it doesn’t have a copyright notice, it’s not copyrighted.”
This was true in the past, but today almost all major nations follow the Berne copyright convention. For example, in the USA, almost everything created privately and originally after April 1, 1989 is copyrighted and protected whether it has a notice or not. The default you should assume for other people’s works is that they are copyrighted and may not be copied unless you know otherwise. There are some old works that lost protection without notice, but frankly you should not risk it unless you know for sure.