vBulletin Manual - Introduction to Scheduled Tasks

Scheduled tasks allow you to run scripts at specific intervals. It is essentially equivalent to cron jobs’ in Unix and scheduled tasks’ in Windows®.

Examples of things you might use scheduled tasks for include daily and weekly email digests and automatic, happy-birthday emails.

Warning:

vBulletin’s scheduled tasks system does not constantly run on the server like cron and scheduled tasks in Windows®. Tasks are automatically run when a user browses your board, so if your board does not have any traffic at a given time, tasks scheduled to run then will not run until later.
vBulletin’s scheduled tasks also will not run when you board is turned off.