What's the difference between this module and shib_auth?

The shib_auth module integrates your Drupal site with the Shibboleth Service Provider, allowing users that have Shibboleth sessions to be single-signed on to your Drupal site.

shib_auth replaces Drupal's normal login process and allows users to be authenticated users via Shibboleth. This lets your Drupal site be a service available to Shibboleth users.

The shibboleth_drupalauth module integrates with the Shibboleth Identity Provider and allows Drupal to be the authentication (and/or session) source for the Shibboleth IdP.

shibboleth_drupalauth replaces the Shibboleth IdP's normal login process and allows users to be authenticated by Shibboleth using their Drupal credentials. This lets users that are logged into your Drupal site single sign-on to remote Shibboleth services (which could be other Drupal sites with the shib_auth module configured.)

If you need to integrate Shibboleth and Drupal, you're probably looking for the shib_auth module.