• Copyleft software - free software whose derivatives and copies must also be distributed as free software.
  • Non-copyleft free software - Free software whose license allows derivatives to be turned into proprietary software.
  • Proprietary software = non-free software
The Free Software Foundation follows the rule that we cannot install any proprietary program on our computers except temporarily for the specific purpose of writing a free replacement for that very program. Aside from that, we feel there is no possible excuse for installing a proprietary program.
  • Freeware - generally refers to software that is distributed free-of-charge, usually in binary. Not the same as free software.
  • Shareware - allows users to share applications with others, but it does not include the source code and usually requires users to pay a fee if they continue using it.