Cron running by itself? My Drupal 7 instance was getting slow. I realised that the problem was a cron job running. However, why is cron running when I'm browing the site? It shouldn't be, surely? It should only run when I run cron.php. 

I don't have poormanscron running or anything else that I am aware of that could be kicking off cron. Any ideas of what could be causing this on Drupal 7?
In Drupal 7 cron is run by Drupal by default.

You can configure it at: admin/config/system/cron

It is preferable to disable that cron and run it automatically from the server in some way.
That method is really only preferable if for any reason you have no other choice.

That said, you really should be automatically running cron regularly.
Not necessarily every hour but at least once a day.