The size of a web page is around 10 to 50 kb - most of the disk usage is graphics. If you throw in large image galleries, big databases, scripts, email and log files, the total size can easily rack up.
Many web hosting providers offer one email account with their free web hosting plans. Even if only one person operates the website, it is convenient and logical to have separate email accounts for different purposes. For example, you can have firstname.lastname@example.org for private emails, email@example.com for general inquiries and firstname.lastname@example.org for signing up newsletters. It is also nice to get email aliases.
Also check if the web host offers webmail with your free web hosting service. If not, you will have to download your email to your computer with a POP3 mail client (ie. Outlook).
Data transfer is the amount of data transferred to and from your web hosting account. This includes file uploads, emails and visits to your website. Normally, visits to your web site will be the dominant factor.
If your average web page size is 20kb, 50,000 page views will result in 1 gb of data transfer. Most web sites use less than 1 gb per month.
Many free web hosting providers do not offer this, but it is a very cost-saving feature to have, especially if you're planning on running multiple websites under one account.
If your free web hosting provider offers multiple domain hosting, there is normally a limit to the number of extra domains you can host with one account. However, the apportioned server resources (disk space and data transfer) of a free web hosting account, you normally cannot host more than a handful of websites on one account.