For years now, programmers have been writing and sharing programs without charge. The internet is a matchless distribution medium for such programs, which may have been written to fulfill a particular need, or just for practice.
Rather than getting involved in the headaches of trying to make a profit on these programs, some programmers decide to freely release their works to any and all who might find them useful. The Free Software Foundation's "GNU General Public License" is a (relatively complicated) example of this. Linux is an example of a complete computer operating system developed and shared under such an arrangement.
Other programmers, hoping for slightly more contact with end users, evolved the concept of postcardware. Anyone is allowed to use the software without payment; those who use it are morally obligated to send a postcard to the author, so the author can
n. Shareware that borders on freeware, in that the author requests only that satisfied users send a postcard of their home town or something. (This practice, silly as it might seem, serves to remind users that they are otherwise getting something for nothing, and may also be psychologically related to real estate `sales' in which $1 changes hands just to keep the transaction from being a gift.) (Jargon File 3.0.0)Link to The Jargon File version 4.x (HTML version).