Atom
In plain terms
An XML format for web feeds, an alternative to RSS.
Going deeper
Atom standardizes syndication feeds so readers can subscribe to website updates, addressing some RSS shortcomings. It distributes content updates.
Under the hood
More rigorously specified than RSS; used for blogs, podcasts, and APIs (AtomPub).
Examples
Blog feeds, podcasts, content syndication, feed readers, AtomPub.
Think of it like...
Like a standardized subscription format for getting a site's latest posts.