Details of trust system: https://bitcointalk.org/index.php?topic=211858.0

In short, theymos sets DT1 members, and DT1 members each have their own DT2 list. If you are on DT1 or any of the DT2 lists, you are on "DefaultTrust" which is the default trust system every account starts off with. However, every member on the forum has the option to exclude/include anyone from their own individualized trust list. But by default, everyone starts off trusting DT1 and DT2 members.