Sub domains are separate folders within your server space, and you can have as many sub-domains as your host will allow. Imagine that you have the domain name:- mags100.com
The .com part is the first level domain
The mags100 part is what's called the second level domain.
If you created a sub-domain called: pcadvisor the url of the subdomain would be: pcadvisor.mags100.com and that would be referred to as the third level domain.
To finalise:- The full address of the sub-domain would be www .techadvisor.mags100.com (space inserted to stop the forum making a 'click here' hyperlink) and you could have an index page inside the folder if you wanted to design a completely separate website at that address, with as many pages as you like. You can only have one index.htm file however - all the others must be something.htm.