My site is powered by WordPress, which has functions that automatically output several different RSS feeds.  An RSS feed essentially allows a person to read the posts on my site without actually visiting my site.  Using an RSS reader (written about here and here), a person can view content from several different websites all in one place, without actually visiting any of those sites.  This saves time and generally makes things more convenient. 

WordPress automatically generates several different feeds:  RSS 2.0, RSS 1.0, RSS 0.92, and Atom.  These formats are all essentially the same, though they have slight differences.  The differences are sort of like the differences between Microsoft Word 97 and Word XP.  They're not that big of a deal, but if you had the choice, you'd probably go with Word XP.  With that in mind, I'd recommend using Atom, followed by RSS 2.0.  These are the most recent in terms of development, and Atom is the only format that's still in development.  This means that there will probably be an Atom 2.0 sometime in the future, but that's not something you need to worry about.  RSS 2.0 is copyrighted by Harvard and its development is frozen.  More info about the differences between these two formats can be found here. 

In addition to these site feeds, WordPress also generates several more specific feeds.  Each individual post has its own feed, which basically tells a reader when the post receives new comments.  For example, my "Intolerance of lactose" post (which had a surprisingly large number of comments) has it's own feed.  There's also a feed for all comments on all posts, which was recently updated and written about here.  There are also feeds for each category (such as the food category) and each page (such as the about page).  The bottom line is that WordPress generates an obscene number of feeds which have little to no actual purpose.