APC caching with Drupal I am confused how APC caching hooks into Drupal system or vice-versa.What I knew so far is APC caches the opcodes for the currently being executing PHP file so next time the various compilation steps can be avoided and then opcodes are served directly from RAM. Since a Drupal page is built with various functional blocks like blocks, node, I am not getting how APC helps it serving fast.I know it does but how?
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 :)