• Free software - permission for anyone to use, copy, and/or distribute, either verbatim or with modifications, either gratis or for a fee.
  • 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.
  • Open source software - more or less the same collection of programs as free software with some exceptions.