Both your diskspace (which includes your website, email, databases, etc) and your bandwidth (amount of data you transfer per month) are limited by your hosting plan, when you go over these limits you will be sent an overage email.
Diskspace Overage - In the case of a diskspace overage, you have two options. Either clear down your used space or purchase some more from (see link below). Please note that email storage is put against your total quota and does not run on a seperate meter.
Bandwidth Overage - Unlike diskspace overage, once you have gone over your alloted bandwidth limit, there is nothing you can do that month to bring it down. Generally, if you just had a brief spike or were only a little over, we are happy to let this slideÂ - however if you continuously go over your limit, you will need to purchase a bandwidth upgrade (or face overage charges).