Atom is a format to publish content in XML to be used by feed readers and news aggregators. This is known as the Atom Syndication Format. WordPress can automatically generate an Atom feed for content and it can be included into the header template to be automatically discovered by feed readers and news aggregators. These feed readers regularly check for updates on your site and then allow their users to keep up with your new content. RSS does the same thing, but Atom was developed to fix some of the flaws in RSS. These web feeds are found on many large web sites and even some smaller ones. Many times they let users choose between the RSS or Atom format.
The feed readers are typically standalone programs that run on the client’s computer but browsers have been moving towards having their own integrated feed readers. The advantage of this is that the feed reader requires no installation and the feeds are available on any computer that can access the internet. Sometimes these feed readers can even combine numerous web feeds on a certain topic.
This term can also refer to the Atom Publishing Protocol which is a related web standard and is often called AtomPub. This protocol allows you to publish, modify, and delete posts and media files remotely. Starting with WordPress 3.5 AtomPub is not activated by default but the Atom Publishing Protocol Plugin can be used.