What is a feed?
XML is a powerful technical specification but powerful specifications are nothing without a useful implementation. RSS and ATOM are two such examples of an implementation that is both fun and useful. However, that means nothing if you don't understand what they are for.
Atom and RSS are site feed formats. A site feed is simply the raw content (headlines, story etc) of a site. While this sounds rather dull there is plenty that can be done with it.
For example you can head your favorite Thanet blog on your PDA, mobile, desktop or even in some email clients (like Thunderbird). It allows headlines from lots of sources to be pulled together into a giant custom super blog of everything you are interested in or push out from your blog and into headline aggregation sites for others to discover.
If you use the browser Firefox you can use a feed to have live up-to-date bookmarks of the latest items for any site that produces a feed. If you use any of the many specialist start page providers (my Yahoo, Google home page and the like) you can pull the headlines in and have them there for you to read in the morning.
Many bloggers use them to keep tabs on the vast numbers of other bloggers whose blogs they watch. You do not even need to know how it works because most modern readers will do all the leg work for you.
Rather than go over old ground again I will point you to some some great further reading if you happen to find this topic interesting.