I believe all the existing answers are mistaken. The itms protocol is used when a podcast is not listed on iTunes at all. It's used as a shortcut to subscribe a regular RSS feed that isn't on iTunes. It's a way to allow someone to subscribe to iTunes without having the feed listed in the directory.
It works simply by replacing the http:// protocol prefix of an RSS feed URL with itms:// So, if you'd like to get the regular RSS feed of a show that's linking to itms, just replace the itms with http.
Here's an example: A show I help produce is called Keith and The Girl. Their RSS feed is http://www.keithandthegirl.com/rss Their iTunes link listing is https://itunes.apple.com/us/podcast/keith-girl-comedy-talk-show/id253167631?mt=2&ign-mpt=uo%3D4
However, you can subscribe to the RSS feed directly by replacing the http in the RSS feed url with ITMS like so itms://www.keithandthegirl.com/rss (SE won't let me make that link clickable but you get the idea)
So, now you see that ITMS and HTTP are interchangeable. Whenever you encounter an ITMS link, just replace it with HTTP.