There are two very good, but very different, ways to generate publicity for a podcast. Podcast networks such as Podshow, The Podcast Network and the Tech Podcast Network provide a place for listeners to find shows that fit their interests, but the selection is generally pretty slim. Directories, on the other hand, are literally overflowing with podcasts in all genres and topics. There's nothing stopping podcasters from using both methods, but the distinction between the two is important and can have a great impact on expectations.
Networks will tell us that their raison d'etre is to provide shows with a minimum guaranteed level of quality, competence, and frequency. Potential listeners that visit a network and find a show they like can be satisfied that the show hosts are relatively knowledgeable, the show audio quality will be decent, and episodes will come out on some kind of regular schedule. Since networks are generally an 'apply for membership' proposition, being accepted to a network has some panache surrounding it and will do good things for a podcaster's credibility.
Being accepted into a network is also a pretty good way to become a star overnight. My very first episode of the GNU/Linux User Show on The Podcast Network garnered well over 2,000 listeners and at the show's peak I routinely had 5,000 or more listeners. I have never had any of my independent podcasts go over 1,0000 listeners.
Some networks take care of all the technical aspects of the show such as the show blog, disk space, and RSS subscription feed. Podcasters doing it alone will generally have to provide all of that themselves. Being taken care of isn't guaranteed, though. I know of at least one network that requires podcasters to provide their own hosting and feeds.
Money: there aren't many podcasters making money yet, but there will be. Advertisers are much more inclined to spent their budget on a network that has a good breadth of high quality, frequently produced shows than they are on a single podcaster with a single show.
A network show is just that: a network show. In most cases, podcasters will not retain ownership of the show when and if the relationship with the network terminates. This isn't true in all cases, but podcasts are inventory to a podcast network and most aren't likely to let a podcaster walk out the door with their inventory. Attention to the contract is important before signing on.
Networks exist to generate as large an audience as possible. Therefore, the subject matter of networks is generally pretty watered down and limited only to popular topics. Networks are more like main stream media than citizen media because many of the networks have no interest in carrying niche shows which, arguably, is the biggest strength of citizen media.
Free for all sign up. There is no application process for most directories. Individual podcasters sign themselves up and appear in the directory.
Ownership: Directories make no claim on the ownership of any show. The individual podcaster retains complete ownership.
Control: Along with complete ownership, the podcaster generally retains complete control over the show content and frequency. Some directories will insist on the show having a maturity rating so listeners know when to expect foul language and such, but generally any topic inside the law is allowed.
The free for all sign up. Directories make no claims to the quality, frequency, or content rating of their shows. Individual podcasters just sign themselves up. Listeners sampling new shows have to fend for themselves and dig through the coal to find the diamonds. Many give up and go to the networks.
The decision about whether to list a show in a directory (or all of them!) is pretty easy. There's no real down side to doing so and each directory that a show appears in offers a little more exposure. The network decision is a little harder to make, however. Podcasters that are considering joining a network should read the agreement very carefully and consider the ownership clauses. If they're giving up ownership to the show to the network, that has to be weighed against what ownership means. When the ABC anchorman retires, the ABC news doesn't go off the air. A new anchorman shows up and carries on. Podcasters should think about how they would feel about someone else doing their show once they've moved on.