Atom is a standard for syndicating content, very similar to RSS. The differences are that RSS has a channel with items, Atom has a feed with entries. Yes, it sounds about the same, and it's very similar. They have different element names that do about the same thing. They use different date and time formats. Atom was designed to fix some perceived problems with the RSS spec. But just like BetaMax was better than VHS, VHS had already gained ground and ultimately won. So RSS got mindshare before Atom. Podcasting uses an extended RSS format. Most blogs will produce both RSS and Atom, so it's up to you which you'll use. Google is a large bastion of Atom, as that's what YouTube and Google Calendar use. And.... reddit switched from RSS to ATOM in January 2016, so we'll see if it's getting its second wind.
Ugly but useful article by Sam Ruby, who started Atom in 2003