'global_start' is a hook location where you can place PHP code that will be ran on every page - You can add a hook to it in the AdminCP under 'Plugin System > Add New Plugin'
I know I'm being really stupid - but I don't see a 'plugin system' option down the left column, or in the 'vBulletin options' section
I am looking in the right place, right...there should be a 'plugin system' bit on the left column which I click to expand more options and 'add new plugin' should be one of those options?
-Ross
[edit]
I just found this;
'From version 3.5, vBulletin has an extensive plugin system, allowing new features to be added and functionality to be changed by third-party add-ons, without modifying the core vBulletin code.'
I am using vBulletin 3.0.8, is there a similar option for me?