The disk space function displays the overall volume of information that you are able to have on the web hosting server at any given time. With a home PC, for instance, this would be the size of your hdd or the overall size of all the hard drives in case that your computer has more than a single one. The same way that your space on a home machine is shared between installed applications, docs, music files etc, the server storage space is ordinarily shared between internet site files, databases and email messages. Every file, folder or e-mail uses some storage space on your server, so you should take into consideration quite a few factors, not only the size of the files you upload. For example, receiving larger email attachments or running a script-driven site where the user-generated data is kept in a database will also affect the space you use.