THX is primarily a quality assurance program. THX certification guarantees that a piece of equipment meets certain minimum standards, which vary depending on the certification level. Most, but not all, of the details are kept secret, although some have been deduced. (THX Ultra amplifiers have to be stable when driving a 4 ohm load, for example.)
The THX requirements for speaker placement are well documented. See http://www.thx.com/consumer/home-entertainment/home-theater/surround-sound-speaker-set-up/
In addition, there are a few sound manipulation features which THX specifies. e.g. Re-EQ, Timbre matching, Loudness-Plus, etc. (See http://www.thx.com/consumer/thx-technology/thx-av-receiver-features/
Certification costs money, so many manufacturers are not bothering to get their equipment certified these days. Many customers argue that the price increase caused by THX certification isn't worth it, either. Others disagree and are willing to pay the premium in order to have the reassurance that the equipment they've bought is high quality.
A little more information is available on the THX Web site at http://www.thx.com/
and at http://en.wikipedia.org/wiki/THX