What is the meaning of "#" or "#!" which appears on url's website

Question

When browsing some popular websites, I noticed that sometimes the URL will change to include these “#” or “#!” Characters. Why is this? From the point of view of programmers, what methods, technologies do they implement to cause this. Is this the best practice for a particular solution?

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

Answer 1

Here is a detailed explanation of hash urls and hash bang: HashURIs by Jeni Tennison.

Answer 2

. ( ), .

( :-)) http://www.w3.org/TR/html4/struct/links.html#h-12.2.3

javascript/ajax, facebook, twitter .. ! #, javascript .