Leo Laporte of This Week in Tech best defines podcast

Netcasting (also known as “podcasting”) is a great new way of distributing audio automatically. When a site (like this) offers a netcast feed you can subscribe to the feed using netcasting software and any new items will be automatically downloaded to your computer. If you have a portable audio player, the files can also be copied to the player. However, and this is a common area of confusion, no iPod or other MP3 player is required (that’s one of the reasons we prefer the term “netcast” to “podcast”). You can listen to the audio on your computer just as you would any MP3. In fact, with our feeds you don’t even need special software. Just click the feed link and download the show you want directly, or use the player built into every page. If you decide you want to listen to a show every time it comes out, you might consider using a netcasting or podcasting client. Think of it as Tivo for Internet audio. You subscribe to content you want by visiting the web site and getting the netcast URL. Your netcast software automatically downloads any new shows and copies them to your MP3 player. There’s always something new and interesting to listen to and you never have to check the web to see when a new show is available. By far the easiest way to subscribe to netcasts is to install the free iTunes program for Windows or Macintosh.