We want you to get content on your terms, so we publish full feeds, not excerpts. The iTunes podcasts will auto-subscribe in the iTMS.
A web feed (or news feed) is a data format used for providing users with frequently updated content. Content distributors syndicate a web feed, allowing you to subscribe to it. Making a collection of web feeds accessible in one spot is known as aggregation, which is performed by an aggregator or a feed reader.
For more info: Feedburner.com offers a good introduction to feeds, readers and content syndication, Wikipedia has an article on web feeds, or you can watch RSS in Plain English on YouTube, which is a great way to learn about feeds.