.htaccess code ?
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What is the code to redirect
www.xyz.com/abc where abc is a folder to www.xyz.com/abc.html
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Hey Atul
There is a really good cheat sheet here:
http://www.addedbytes.com/cheat-sheets/mod_rewrite-cheat-sheet/
It shows you the most common things you will need to do and is certainly worth printing off and super glueing to the wall next to your desk.

So, looking at the sheet (and you really have to learn this stuff rather than luck it through else you will end up going around the bend when something does not work as you want it to) a basic rewrite works as follows:
RewriteRule ^page.html$ new_page.html [R=301,NC,L]
So, to make this work for you
RewriteRule ^page.html$ new_page.html [R=301,NC,L]
But, that is, assuming I understood your question and you are actually looking to redirect A to B and not to just create a nicer URL by masking the /abc/ as /abc.html in which case this is the basics for creating a pretty URL
RewriteRule ^([A-Za-z0-9-]+)/?$ categories.php?name=$1 [L]
- that is for a more dynamic page with a querystring but the theory is the same.
Maybe if you provided an exact example I can give you one example to build on?
Hope it helps!
Marcus