The little orange button has become almost omni present on most good websites. I have been a syndication junkie for quite some time now. However, the exact difference between the two popular formats - RSS and Atom has been always been unknown to me. All i've heard is that Atom is supposed to be new and better, but RSS started it all and has the bigger following. Frankly, most readers that i've seen support both and so as a user i am impervious to the difference.
However, it was nice to know the difference. Here's Rss20AndAtom10Compared - Atom Wiki with the article. For anyone who holds an interest in various applications of XML, this is a must read.