To answer that question in a straightforward manner, Podcasting is a technology that enables anyone to deliver his or her own syndicated audio content to the world. This differs from traditional means of downloading audio off of the web because this technology can automatically update the content on the computer of someone who subscribes to your Podcast. Apples iTunes is the most popular program for finding and downloading Podcasts. After all, half of the term comes from the name of Apple's "iPod" MP3 player (the other half of course comes from the term "broadcast").
The technology for Podcasting developed as a means to incorporate an audio functionality into blogging, the process of keeping and updating an online journal. With the development of "enclosures" in RSS, a type of xml that allows the syndication of web content, this audio functionality became possible. Basically an RSS aggregator such as iTunes retrieves an RSS feed generated by the software used to post a Podcast, and the content is updated to the subscriber when new content is available. These aggregators are often referred to as "Podcatcher's."
What does this means to the end user who subscribes to Podcasts? It means that ones source of information, music, or any audio content is no longer limited to mainstream or independent, local or national broadcasting. Programs such as comedy, cooking, music, news, or anything one would wish to broadcast, can come from anyone who wishes to Podcast it, and has the technology needed to do so. It has made it possible for anyone to not only broadcast online, but also syndicate his or her own radio program.
Podcasts generally have a structure that emulates that of a conventional program broadcast on the radio. There is usually some catchy music that leads into an intro. For an informational Podcast there may be a main topic that will be discussed in segments. These structures are often guidelines developed to efficiently and coherently deliver content to the listener. There are however no final rules on how to structure your program seeing as how each person has jurisdiction over his or her own content.
While major news stations, television networks, and radio stations may have Podcasts, so does the independent politics enthusiast, local musician, and teacher who provides information to his or her students beyond the classroom.
So by now you're either asking "How do I get a Podcast?" or "How do I make a Podcast?" The answer to both is a simple process. Online you will find several places that host Podcasts, and have directories to them. Many of them will download right into iTunes. With iTunes as well you can search for Podcasts in the "Music Store." (Even though the majority of them are free) When you find one you like, simply subscribe to it and the Podcast will be downloaded right into your music library. From there you can upload it to your iPod, or set your preferences to automatically upload the new content. If you're ready to make your own Podcast there are several places that will host your Podcast, and even some of them are free of charge. Search for blogging sites and see which ones support Podcasting. If you have your own web host and want to host your Podcast at your own site, check with them to see what sort of support they provide for Podcasting, and how to implement it on your webpage. Make sure to find out what sort of bandwidth your account has. If your Podcast becomes too popular too soon, you will want to make sure you avoid extra charges by staying within your bandwidth limit.