They use atmospheric noise to add entropy to their RNG. The definition of "True" in this case is rather loose, but once you get enough bits of entropy you can feed it into a PRNG or conditioners to give the output the qualities you want.
Unfortunately, the source code to the RNG is close-source, and they say they aren't going to release it. But my guess is they use that random noise as a seed to a PRNG. Their FAQ has quite a lot of information though: http://www.random.org/faq/#Q1.2
Here is some history of random.org http://www.random.org/history/
and the wiki page http://en.wikipedia.org/wiki/Random.org
the wiki page for atmospheric noise briefly discusses it's applications to "high quality" RNG http://en.wikipedia.org/wiki/Atmospheric_noise
Unfortunately, the source code to the RNG is close-source, and they say they aren't going to release it. But my guess is they use that random noise as a seed to a PRNG. Their FAQ has quite a lot of information though: http://www.random.org/faq/#Q1.2