Really Simple Syndication (RSS)

Sometimes known as "rich site summary."
A format for delivering regularly changing web content. You can choose to subscribe to an RSS feed so that you are constantly sent updated information. An aggregator (reader) is software that accesses the information and sends it to you when it is updated. 
Web-based aggregators:
Bloglines-One of the early online RSS readers, Bloglines “split screen” interface makes it easy to view the full content of any of your feeds.
Google Reader-Reader keeps track of which things you've read so that when you only see unread items when you come back. If there's a dark blue border around an item, Reader is marking that item as read.