Postcardware, also called just cardware, is a style of software distribution similar to shareware

Shareware

The term shareware is a proprietary software that is provided to users without payment on a trial basis and is often limited by any combination of functionality, availability, or convenience. Shareware is often offered as a download from an Internet website or as a compact disc included with a...

, distributed by the author on the condition that users send the author a postcard

Postcard

A postcard or post card is a rectangular piece of thick paper or thin cardboard intended for writing and mailing without an envelope....

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This is similar to beerware

Beerware

Beerware is a somewhat tongue-in-cheek term for software released under a very relaxed license. It provides the end user with the right to use a particular program .-Description:...

. A variation, Emailware, uses the same approach but requires the user to send the author an email

Email

Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...

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Postcardware, like other "novelty" software distribution terms, is often not strictly enforced.
The concept was first used by Aaron Giles, author of JPEGView

JPEGView

JPEGView was a popular image viewer for Mac OS in the 1990s by Aaron Giles. Initially released in 1991 it was one of the first internet image viewers for Mac OS. The program was also the first postcardware. Giles said he had "received somewhere between 5,000 and 10,000 postcards"...

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