Public Domain Software whose authors renounce any ownership over the software. Since the software has no author, you may do anything you like with it without any payment to anyone. You can even sell it yourself.

Free: Free software requires no payment (although some authors license their work for no charge to individuals and non-profit organizations, while charging businesses). The author retains copyright. Check documentation or the author for details of licensed use.
PD: See Public Domain
Public Domain Software whose authors renounce any ownership over the software. Since the software has no author, you may do anything you like with it without any payment to anyone. You can even sell it yourself.
Shareware: Shareware products are not free. They require payments directly to the author if you continue using them after evaluation. Fees charged by EMS only cover EMS's assembly and distribution costs.