Q:

Different user permisions for different sites on sharepoint

alt text http://img21.imageshack.us/img21/3262/snapshotosz.png
I want to add a new tab or site in sharepoint as shown for the above, but i want to allow some users to access one of them and not the other.  when i remove a user it is removed from both of them or put for all. 
Also how can I edit those of the one which is already done? 

A:

You need to set the permissions of the subsite to not inherit from the parent.  This is an option when you create the site.  To stop inheriting permissions on an existing site, go to:
Site Actions -> Site Settings -> Advanced Permissions -> Actions -> Edit Permissions
Then you can set who has access to the subsite.  A user with no permissions in the subsite will not see the tab. 
It's all explained here.