The .htaccess file is a plain text file that is used by Apache web servers. This file is used to store information about blocking IP, users and sites, blocking robots, handling re-directs to other sites, preventing director listing and password protecting folders. It can also rewrite URLs, in layman's terms that will change the web address seen in your viewers browser.
So.. powerful stuff!
You can check more detailed info over at corz.org - corz knows a thing or two!
If you're really interested about rewriting URLS (using mod_rewrite) then head over to this mod_rewrite beginners guide for a simple step by step tutorial.
Enjoy the power (and remember to make a copy of your .htaccess file before you bugger around with it!)
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