what is .htaccess file and what's the use in seo?

not sure what it can do for seo. but i thinkt its primary purpose is for access permissions to directories in your publich.html

I hope if any one can let me know about this, even i am not sure of this file. Good to share knowledge here!!!

You can rely on this site: http://en.wikipedia.org/wiki/Htaccess

Role and usage of .htaccess are posted.

Not sure what it can do for seo. But have some seo problems solve using .htaccess
ex: redirect none-www to www

RewriteCond %{HTTP_HOST} !^www.domain.com$
RewriteRule ^(.*)$ http://www.domain/$1 [R=301]