What is the purpose of #! in an url [duplicate]

This question already has answers here: Closed 12 years a开发者_运维技巧go.

Possible Duplicate:

What's the shebang (#!) in Facebook and new Twitter URLs for?

This question may have been asked before so bear with me. #! (pound exclamation) is not a great search term neither on this site nor on google.

So anyway, I'm noticing a lot of facebook urls with these two characters in them. What is it used for?

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This should explain it:

http://googlewebmastercentral.blogspot.com/2009/10/proposal-for-making-ajax-crawlable.html

It boils down to making AJAX-sites crawlable.

Update: And from the duplicate: http://code.google.com/web/ajaxcrawling/