THX stands for "Tomlinson Holman's eXperiment," and it is an industry standard for sound reproduction developed by its namesake at Lucasfilm studios.
For speaker systems, THX Certification is meant to ensure that the sound produced by an audio system (most often 5.1 Surround Sound) is heard in exactly the manner that the audio engineer intended, thus making the standard an intensely popular feature for Hollywood sound artists.
THX Certified sound reproduction does not require that audio be recorded in any particular format, whether it is something such as Dolby Digital sound or otherwise. Rather, THX is most important at the moment that the sound is reproduced, or "played" by a speaker system. THX Certified surround sound systems such as 5.1 or even 2.1 multimedia surround sound home theater systems are used to play THX sounds from computers, televisions, and video game systems.