Sometimes when you're researching a given topic you'd like to monitor a specific web page for changes. Usually, pages that change frequently (such as a blog) will provide an RSS feed that you can use to track changes. (e.g., you can use Google's Reader to subscribe to RSS feeds from blogs.)
BUT SOMETIMES the web pages don't have a feed you tap into. That's a pain, because it means you have to remember to go back and check the page periodically.
The latest feature from Google Reader is that you can set your own "custom feed" if you want to be alerted whenever a specific page has been updated.
For example, if you wanted to follow Google.org's latest products, just go to Google Reader (at www.reader.google.com) and type "http://www.google.org/products.html" into Reader's "Add a subscription" field (the button is in the upper left of the page).
After you click that button, it will ask you for a web page to monitor. Click "create a feed", and Reader will periodically visit the page you specified and publish any significant changes it finds as items in a custom feed created just for that page.
For a full description see the Official Google Reader blogpost.