Would like to have multiple sites with common usersnames from one database but completely different content.

I have a client that loves drupal and would like to have quite a few sites that can be selected by a dropdown menu at the top of the page. When anyone selects the different site or location to be more exact. The different location will have completely different content, with the exception of the usernames and passwords.

Each different site needs its own separate installation? Or can there be one database with an option in the admin section for adding multiple locations with like a checkbox when adding content, ads, blocks and other modules etc...

Maybe some how having that as an option, so when I add a block, content page, forum entry, poll or advertiser, I can choose which location I want the information displayed.

And is there a classifieds sections module available that also has the same ability to have different classifieds in each different location.

And if this can be done, just have the front page with a location selection and a standard hompage.

I know I am asking alot but I am very fond of Drupal's scalability and ease of use.