Creative Commons licenses give content creators the option to license their works for others to use for free. This means you can use the content, provided you abide by the license agreement.
Aside from these six licenses. Content creators can also designate their content with a CC0 - this is when they have given up the copyright to the content, allowing it to be reused with no conditions attached.
More detailed information is given in the Creative Commons website.
The four things to include when attributing or giving credit to a creator, are:
Example of attribution
Note: Wellcome Collection provides examples of how to credit images from their collections.