How can I direct the user to a different page on a multisite installation based on which subdomain they go to? I have a site, let's call it www.example.com, and I'm going to create subdomains for it with the same Drupal installation on every subdomain. I can do that. However, when someone goes to a subdomain, I would like them to see a different page depending on which subdomain that they go to. Say for example they go to user1.example.com, can I make that somehow redirect them to an arbitrary page within the Drupal installation? I hope that's clear.
This is one of the things that you can do with the Domain Access module.  Essentially, it allows you to have different behaviors for a site with subdomains, but running from the same database.  This is different from multisite, which uses a different database for each site.