Disk Space
Disk Space is the amount of storage you have for all of your files on your hosting account. Your disk space is calculated based on the files you uploaded to your account, the content of MySQL Databases, and the content stored in your Email accounts on the server.
Monthly Data Transfer
Although it pretty much speaks for itself, your monthly Data Transfer is the amount of data you can transfer to and from the server in 1 calendar month. This includes both in and outwards traffic (upload and download). Monthly Data Transfer is commonly referred to as "Bandwidth" which is an incorrect use of the term. Bandwidth referres to the throughput or "speed" of a connection.
Parked / Add-On / Sub - Domains
A parked domain is where you sit another domain ontop of an existing website. Lets say you own two domain names, domain1.com and domain1.co.uk. You setup your account with a website using the domain1.com domain name, but you also want domain1.co.uk to point to the same website. To do this you would "Park" the domain, using the feature in cPanel.
Add-On domains do something slightly different. They allow you to host multiple sites on a single cPanel account. Let's use the same basic example. You own two domains, domain1.com and domain1.co.uk. You setup domain1.com but want a slightly different site on your .co.uk domain. Add-On domains allow you to point your domain to a sub directory. A basic example is shown below. From looking at the root or top level directory of your account, this is how it would be organised:
Directory Name Domain
/ domain1.com
/ukwebsite/ domain1.co.uk
Subdomains are basically the same as Add-On domains, except you use an addres such as ukwebsite.domain1.com instead of a full domain name.
MySQL Databases
MySQL Databases are used to provide the backend for scrips such as Blogs, Bulletin Boards and content management systems.
