What is Atom in WordPress?

Sitemap can be in any format XML, RSS and Atom feed The main difference between these is that the XML Sitemap submits the details of the URL of all the pages and posts of your website to the Google webmaster.

Whereas the RSS / Atom feed describes all the changes happening in your website and updates it in the webmaster.

XML sites are very large because they keep all the data of your website in url format whereas RSS/Atom feeds are small files as they only update the recent changes.

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.