APC caching with Drupal
APC is PHP's op code cache. If you enable it then it works for any PHP code. All of your Joomla/Wordpress or Drupal sites will execute faster provided you have high value of Hit/Miss ratio( or increase APC RAM if it's less).

If you want further optimization then Drupal can use APC for caching( instead of storing in mysql tables) using Drupal's APC module. I'm not sure about this sentence though.

I too was confused in the same way :)