What are Source Ports?

The Quick Answer

A source port is a fan-made copy of the Doom engine. In other words, you can use a custom program to play Doom, provided you have legal copies of the WAD files it requires.

More Details

In 1997, the source code for Doom was released to the public, under a license that allowed fans to create their own versions of the game's engine. At first, these engines simply made it possible to run Doom on new operating systems, but over time, fans began experimenting with changing the engine, either fixing known bugs or adding new features.

Many modern source ports are not very faithful to the original game - but that's okay, everyone has their own favorite way to turn demons into meat paste.

Two of the more important source ports are Boom and Man's Best Friend (often abbreviated to MBF). The new features, such as conveyor belts and friendly guard dogs, added in these engines have become popular standards in their own right.