Anything that is published online is protected by COPYRIGHT automatically. It doesn't matter who created it. You have to ask the creator for permission before using their work.
If the work is on the PUBLIC DOMAIN it means that the copyright protection has expired or the creator is happy to let others use their work without asking permission OR giving attribution.
If the work is licensed under the CREATIVE COMMONS the creator allows people to use their work under certain terms, called licenses. There are six different licenses - you can find them below.
