DefaultTrust is a list of trustworthy users maintained by theymos; with the default trust depth, people those users trust are also trusted. Anyone with any concerns about this can (and should!) create their own trust list or at least change their trust depth. You can adjust your personal trust settings here.

I personally still use DefaultTrust, because I trust theymos (if you're using this forum at all, you have some trust in him at a minimum), but I check all of a user's trust ratings and the people who created them before making a deal anyway, so I only really use my trust list for a first impression (though I haven't yet disagreed with it).