Public Domain - these refer to works with no individual intellectual property or ownership laws. These may be in the Public Domain for reasons including the expiration of previous coverage by copyright laws, or because copyright laws do not apply to a particular work.
Creative Commons - Creative Commons licenses allow you to share your work and make it freely accessible, while still allowing you to maintain restrictions around what the work is used for or how you are credited. There are a variety of Creative Commons license types.
Copyleft - Copyleft licenses allow every user of a product (e.g., GIMP software) to freely change or redistribute that product, on the condition that the newly shared product is also able to be freely changed or redistributed.
Open - broadly speaking, content which can be freely accessed, remixed, and shared.
Open Access - an international movement advocating for scholarly research to be made freely available online, and without most copyright restrictions.
Open Source - content which can be freely shared and modified, and which also adheres to a specific set of ten principles.