Multi-client capability und data storage
A big difference between CiviCRM and proprietary SAAS offerings is that you decide where to host the system, hence your second question is not applicable.

In terms of the first question, this is possible via 'multi-site' or other ACL approaches, but the question back to you is 'why not just have separate civicrm instances for each client?'.

You can also achieve a nice multi-site outcome using the Related Permissions extension that Fuzion created.