What's the significance of "#" or "#!" that appears in website url's

Question

While browsing some popular websites, I noticed that occasionally the url would change to include these characters "#" or "#!". Why is this? From a programmers perspective, what techniques, technologies are they implementing to cause this. Is this a best practice for a certain solution?

Examples: http://twitter.com/#!/ http://www.facebook.com/#/

Answer 1

Here's a thorough explanation of hash-urls and hash-bang urls: HashURIs by Jeni Tennison.