Atom is an XML-based document format that is used to describe 'feeds' of related information. A typical use is in a news feed where the information is periodically updated and which users can subscribe to. The Atom format is described in tools.ietf.org/html/rfc4287
The Atom module provides support in reading and creating feeds.
See the RSS module for examples consuming and creating feeds.
The Atom URI W3C Namespace
The XHTML URI W3C Namespace