Any apps you have running which require data, e.g. weather, facebook, email will sync every few minutes depending on your phone setup and will consume some of your data download allowance.
They will never exceed your 500mb allowance, but if you surf a lot or use youtube you will hit the data allowance limit quite easily. Check with your provider what happens after that, some will charge automatically (which can be quite expensive), some nothing happens unless you go way over.
Some providers accounts are quite easy to check and see how much you are using every day.