APC is a nice caching plugin for PHP, that will store compiled php code in memory.  This can cause problems when developing your site however - if you make code changes, you must wait until the next APC refresh to happen.

To clear APC cache manually - get the following file:

http://portal.solutionengineers.net/dl.php?type=d&id=1

Upload it to the root of your website.
Rename clearapc.php to something unique - and set the password inside the file.

Then, you can call the file with your browser like so:

http://www.mydomain.com/clearapc.php?pass=mypassword