THX - developed at Lucasfilm in the early 80's, named after either "THX 1138" (George Lucas' first feature film) or the head engineer for the project, Tomlinson Holman (Tomlinson Holman's eXperiment), depending on which legend you choose to believe.
It's not a recording format or anything like that, actually it was originally guidelines for setting up a commercial movie theater. Later, specifications were developed to apply to home gear to try and attain some uniformity with the commercial sound, so that a properly set up home THX system would replicate the sound of a THX-certified theater to a certain extent.
However, many of the most important THX requirements for theaters are impossible for them to control in a home setting, ie. no noise from outside the theater, no noise from inside the theater (air conditioning, etc.), no obstructed or uncomfortable viewing angles, controlled reverberation inside the theater, and proper image brightness. The other specs are insignificant when compared to these. Therefore, one could argue that THX certification of home audio gear is nothing more than a marketing ploy, since the electrical specs are acheived by the vast bulk of the gear out there, THX cerified or not.