These programs allow for parsing of XML data with specific tags, designed to display simple information from a web server - often a summary of the latest stories or articles of the day. Can also be used to link against hosted audio files to create podcasts, which are then treated as a series of episodes. There exist a number of useful classes within Cocoa that, when tied together, allow a programmer to easily create a news reader. This is a good idea for a newbie to try, since it’s relatively simple compared to an app like Keynote, yet covers more than a simple text editor - think “intermediate Cocoa”.
See more info in the wikipedia article: http://en.wikipedia.org/wiki/News_aggregator
For an example of a feed you could check out in your aggregator, see CocoaDevRSSFeed.
Here is a list of the more popular readers, made in Cocoa on the Mac, so far:
Vienna, open source, Cocoa: http://sourceforge.net/projects/vienna-rss
Postino ( http://www.coladia.com/postino )
… and here is an interview with the authors of these apps: http://www.drunkenblog.com/drunkenblog-archives/000337.html
Some of these apps also support Atom in more recent versions.
An article (though rather old) on O’Reilly also covers some of these: http://www.macworld.com/2005/02/reviews/rssreaders/
MiNews is also another (free) RSS Reader (http://www.js8media.com )