Amnezia said it all, but just to put things in perspective, cron's on *nix are similar the scheduled tasks on windows. the cron simply runs a task at a given time with various options.
PHP has a Command Line Interface (CLI) and the cron just executes your scripts via CLI when called.
by the way, check this out for a thumping list of Ajax uses:
http://www.fiftyfoureleven.com/resou...quest/examples