They use a single level hierarchy of topic categories. The counts of podcasts are pretty small, and I think that makes a single level hierarchy work. They have a "Suggest new subcategory" button so they're thinking ahead and giving the navigational structure a chance to grow in a way that makes sense to the user base.
It's free for anyone to list a podcast in the directory.
They seem to only post a podcast description, and do not post listings of the episodes nor do they help the user listen to the show nor do they help the user subscribe to the show. Hurm.