Lines can be drawn:
Public domain. This is completely open.
Free to use and modify and sell but must retain author's name internally. (BSD-ware)
- Optional clause: Must not advertise that it contains content made by the author. (Since the author didn't oversee the modifications, the author isn't associated with the final product.)
Free to use and modify and sell but must redistribute the original material. (GNU-ware) GNU is not free; it is viral and infects anything it touches with it's own license restrictions.
Free to use for a limited time and share with others, but not modifiable. (Share-ware)
Free to use but not modifiable. (Free-ware)
Pay to license source code and modifiable. (Commercial open source)
- Several Operating Systems are distributed like this for example, because it is too difficult to distribute a binary of an operating system which can link with custom software. ThreadX RTOS, Nucleus RTOS come to mind.
Pay to use but not modifiable. (Commercial-ware)
Pay to use and extendable but not modifiable. (Commercial-ware with third-party plugins)