Shareware

Shareware is a software marketing concept where an incomplete or time-limited version of a program is released to entice people into buying the full version. Doom and Heretic both have shareware versions, each containing only the first episode of their respective games (Knee-Deep in the Dead and City of the Damned), making nine levels each.

Part of the reason Doom II was not given a shareware release, according to John Carmack, was that "A lot of people consider themselves to have "finished DOOM" when they just finished the shareware episode."

For more information on shareware, read its Wikipedia article.