http://wiki.answers.com/Q/What_do_the_abbreviations_mean_on_an_ultrasound_scan

In above url, it appears like "What_do_the_abbreviations_mean_on_an_ultrasound_scan" is really a directory!! however i think its an online directory !!

I wish to learn about these items !! how you can do this stuff and the things they call ???

They're most most likely using mod_rewrite to rewrite the url. A great resource may be the tag wiki for mod_rewrite here on StackOverflow.

The guidelines for mod_rewrite is saved within the .htaccess file, and appears something similar to this:

RewriteRule ^/Q/(.*?)$ answers.php?slug=$1

This could rewrite all demands whitch match a url beginning with /Q/ to solutions.php, and supply whatever is after (within this situation What_do_the_abbreviations_mean_on_an_ultrasound_scan) like a GET parameter. This is available in the script as $_GET['slug'] when they were running PHP.