Many Drupals

Multi-site

/
  sites/
    default/
      settings.php
    www.drupal.edu/
      settings.php
    biology.drupal.edu/
      settings.php

sites/sites.php

$sites['www.drupal.edu'] = 'main';

$sites['fapi.api.drupal.edu'] = 'fapi';

$sites['multisite.building.drupal.edu'] = 'multisite';

$sites['fundraising.drupalassociation.com'] = 'da_fundraising';

/
  sites/
    sites.php
    default/
      settings.php
    www.drupal.edu/
      settings.php
    fapi/
      settings.php
    da_fundraising/
      settings.php

If you want...	Probably means...
Multiple core versions	Separate installs
Different content types	Separate installs, multi-site
Single user base	Domain Access, OG, Single-site
Subdomains	Separate installs, multi-site, Domain Access
One domain	OG, Single-site
Simplified server admin	Domain Access, OG, Single-site
Shared content	Domain Access, OG, Single-site
Per-department ACL	Single-site, OG
Department autonomy	Separate installs, multi-site (less so OG)
Per-department Views	Separate installs, multi-site (less so OG)
Staggered migration	Separate installs, multi-site, Domain Access, proxy server?
Per department theme	Separate installs, Multi-site, Domain Access, OG
Inexperienced site admins	Separate installs, Single-site