A podcast is a sound file that is distributed over the Internet, on demand or by subscription. Podcasts are usually published as part of a series, which is also called a podcast channel. Because podcast episodes can last anywhere from one minute to many hours, they are usually streamed while playing. Podcast episodes, which are also called items, are usually delivered in the mp3 file format. Video podcasts are also popular, but this sample application plays only audio podcasts that use mp3 files.