RSS is nothing to look at. Really. It's a file format that a website or web service uses to make a summary of content items available to the world. Think of it like a listing of upcoming programs on your cable or satellite service. It doesn't communicate the full plot of the program, just enough to hopefully make you want to tune in. RSS is like that. A website will make available an RSS-formatted listing of recent articles with a brief summary of each, hopefully enough to make you want to read more. RSS stands for Really Simple Syndication. Syndication is making the content of your site easily available to any other site in the world. But just like syndicated newspaper columns, just because you publish it, it doesn't mean that anyone will actually pick it up and read it. That's where you have to write compelling text. RSS just deals with the mechanics of passing summaries out to whoever asks.

iTunes was one of the biggest boons to getting podcasts accepted. Podcasts are episodic audio productions that people listen to whenever they want. How does their podcast listening software know when there's a new episode? You guessed it, via RSS feed. When you submit a podcast to Apple, you tell them the url of your RSS feed so your listeners can find new episodes. It's RSS 2.0, with a few additional fields. OK, a LOT of additional fields for things like: category, image, duration, keywords, subtitle, and summary. The industry was then left scratching its collective head asking "so what's the difference between description and summary?" Nobody quite knows. The bottom line is, if you're going to be playing in Apple's sandbox, you got to play by Apple's rules. If you're going to submit a podcast to iTunes your RSS feed should have these extra elements.

The iTunes RSS description

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.

References:

Wkipedia

Ugly but useful article by Sam Ruby, who started Atom in 2003