Atom is simply an alternate XML format for easily sharing content, much like RSS. Blogs on http://www.blogger.com/ can now publish Atom feeds. RSS Bandit recently added support for the most recent version of Atom, so from the user perspective, the details of a feed's format are taken care of for the most part.
Atom and RSS have very similar uses and it may be of benefit to read "What Is An RSS Feed" if you haven't done so already in order to understand Atom better.
The motivation behind Atom sprang from a desire to add improvements to the RSS specification. Since the keeper of the RSS specification froze the specification (declaring that the RSS 2.0 specification would be the last version of RSS), the authors of Atom felt that an entirely new format was necessary. Hence the Atom format.
If you're interested in reading more about Atom, please see the following links: