Google Reader is an RSS Feed Agregator. It allows you to be notified (similar to email), when a web site or blog is updated. This way you don't have to go to the web site every few days to see what has been added. You just log into Google Reader (like you would for an email account) and it will display any updates on sites you have asked it to track for you.
Go to http://www.google.com/reader and sign up for a free account.
I've actually found Google Reader, Gmail and Google Docs all very useful.
P.S. - RSS stands for Really Simple Syndication