Creative Commons (CCL) is a licensing system set up to help image creators distribute their work.
An image has a licence attached indicating how it can or cannot be used.
There are four elements to the licensing system and this information can be found alongside the photo on the website if a CC licence has been applied.
The four elements are applied in different variations to form six licences.
The latter is an overall public domain licence (CC-0) which means the image has no copyright attached and is free to use on all accounts.