[ Help ] The Algorithm used with md5 passwords

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Hello all,
iam working these days on php web application related to vBulletin , and so I needed to make a memberships system, But I don't want the visitors to register again .. ( I want to use the users registered in my forum )
So, i had some problems with the login system .. vBulletin doesn't use the usual md5 system .. I think it adds somethings to it .. i tried to figure out how it works .. but i couldn't really ..
so, please can anyone tell me how vBulletin trat the password since it's input by the user from the login box .. till it's stored in the database ?
and let's say .. that the user's password is aa .. and he is already typed it in the text-box .. and clicked the login button .. what steps will vBulletin take on the password to store it in the database ?

Thanks,