Do you need to share content between the various sites? If so, you'll
probably want to use Drupal's Domain Access module (as you've tagged this
question). However, if these are all going to be distinct sites that do not
have to share content (except maybe through Aggregation or Feeds) - you can
avoid that module.
I'm pretty sure the approach you want to take is documented here: https://drupal.org/node/2622 - basically, you use one
database, but each site uses its own table prefix to distinguish it from
the other sites.
As far as running multiple sites from one Drupal root, you can use the
built-in multisite support: https://drupal.org/documentation/install/multi-site . I
don't think you will need drupal domain access at all in this case.