Podcasts are normally audio recordings on a subject matter, or time shifted radio shows from commercial and professional stations, or promotional material, or extra content for a TV show. To be honest they could be anything. The definition of podcast is really the way it's distributed (via RSS feed).
A lot ares non-professional, just a few people enthusiastic about something. Some are semi-professional - ie experienced in audio production, and have high quality kit, and of course Radio and TV stations like NPR and The BBC producing high quality professional shows.
iTunes is just one podcast feed "aggregator", picking up the feed periodically, and making the latest media available to download, rather than hitting the shows websites on the off chance that there is new content. Other aggregators include Juice (mac/windows/linux), and Pocketcast (for iOS)
Most podcasts are usually under creative commons licenses giving the podcast episodes out as a free download for people to hear and share.
There are a few video podcasts too, usually called vodcasts.
The podcasts I listen too are mainly music based, playing unsigned, independent music from around the world, that most people would not normally hear.
Examples are
http://www.darkcompass.com
http://www.musicpodcasting.org
http://www.madeintheukshow.co.uk
Im sure there's a podcast that suits your interest, there are 100's of thousands of them now.