How do you get blurbs from a website?

When you type in a URL on websites like Facebook and Digg, they automatically pull up a blurb of the website that contains the first few sentances of the site.

For example, when typing in cnn.com, the CNN blurb "Breaking news U.S., World, Weather, Entertainment & Video News" and the first few sentances on the CNN website automatically appear as well.

How is this done? Is there some sort of crawler that goes to the link provided? Thanks all.