Reading lists are OPML documents that point to RSS feeds, like most of the OPML documents you find, but instead of subscribing to each feed in the document, the reader or aggregator subscribes to the OPML document itself.
"This was the final session in my undergraduate Online Journalism module ... taught last May. It’s a relatively brief presentation, just covering some of the possibilities of mashups and RSS, and some tools. The majority of the class is taken up with students using Yahoo! Pipes to aggregate a number of feeds."