htaccess uses redirect to look for any request for a specific page (or a non-specific location, though this can cause infinite loops) and if it finds that request, it forwards it to a new page you have specified:
Redirect /olddirectory/oldfile.html http://yoursite.com/newdirectory/newfile.html
Note that there are 3 parts to that, which should all be on one line : the Redirect command, the location of the file/directory you want redirected relative to the root of your site (/olddirectory/oldfile.html = yoursite.com/olddirectory/oldfile.html) and the full URL of the location you want that request sent to. Each of the 3 is separated by a single space, but all on one line. You can also redirect an entire directory by simple using Redirect /olddirectory http://yoursite.com/newdirectory/
Using this method, you can redirect any number of pages no matter what you do to your directory structure. It is the fastest method that is a global affect.