Terminology
Public Domain – the state of belonging or being available to the public as a whole, and therefore not subject to copyright. Legally copied, modified, package, even if selling it as long you give credit.
Learn more about Public Domain
Royalty Free - or RF, refers to the right to use copyrighted material or intellectual property without the need to pay royalties or license fees for each use or per volume sold, or some time period of use or sales. You may have to purchase the image/video/music but once buy it, don’t have to pay royalties.
Stock Photos - is the supply of photographs, which are often licensed for specific uses. It is used to fulfill the needs of creative assignments instead of hiring a photographer, often for a lower cost. Similar to royalty free but buy it for a specific use with a specific time limit or number of copies.
Creative Commons - is a non-profit organization devoted to expanding the range of creative works available for others to build upon legally and to share. The organization has released several copyright-licenses known as Creative Commons licenses free of charge to the public. These licenses allow creators to communicate which rights they reserve, and which rights they waive for the benefit of recipients or other creators. Started by a group of artist a collection of images, sound and video. Each image says what you can or cannot use it for.
Copy Left - an arrangement whereby software or artistic work may be used, modified, and distributed freely on condition that anything derived from it is bound by the same condition. You can take my stuff and modify it, etc… But then you have to do the same, so when you publish it you have to allow anyone to do the same.
GNU General Public License - (GNU GPL or GPL) is a widely used free software license, which guarantees end users (individuals, organizations, companies) the freedoms to run, study, share (copy), and modify the software. Software that allows these rights is called free software and, if the software is copylefted, requires those rights to be retained.