Feeds, RSS, RDF, Atom, XML... Why so many different terms to refer to what is essentially the same thing?
There's no good answer.
RSS
Some say it stands for "Really Simple Syndication" while others say its stands for "Rich Site Summary" or "RDF Site Summary." Others say it doesn't stand for anything.
There are as many as nine different versions of RSS. But for most purposes, RSS can be thought of as two distinct formats or perhaps a single format that comes in two basic flavors.
RSS is wildly popular, used by countless people all over the world. Some say it will be the "next big thing." Some say it already is. The format is also enmeshed in a tangled web of accusations and personalities.
RDF
Two (or three?) versions of RSS incorporate a standard known as Resource Description Framework or RDF. It's a framework for describing and sharing metadata about Web pages (and other things probably).
Atom
First it was called Pie. Then Echo. Now it's known as Atom. It began development in 2003, when critics of RSS decided to develop an alternative format. At the time of this writing, Atom version 0.3 has been taken under the wing of the Internet Engineering Task Force.
Though not as widely deployed as RSS, Atom is gaining ground and might become enormously successful when it reaches maturity. Atom got a major boost when Blogger decided to use it for their popular blogging software. Google owns Blogger, so the format has the support of a major player.
XML
Both RSS and Atom are implementations of XML, or "Extended Markup Language." But XML does many other things, so the little orange buttons that say "XML" are misleading. It's like naming a link to this webpage "English" — technically true, but not very descriptive.