A podcast is essentially a digital radio show. You can interview guests, talk about current events, and dig out your own niche topic. Podcast files can be stored online to stream or can be made available to download through a variety of programs.
It can be as simple as recording a sound bite on your computer and uploading it to YouTube or to a podcast-specific site, or it can be as complicated as using a professional recording room with expert equipment. Programs like Audacity and GarageBand can be used to splice, cut, and edit the file.