Hi again waspman,
No worries...it takes a little getting your head around when you start out 
As Chris confirmed, the root domain is waspkilluk.co.uk.
The most common subdomain is the www, which is standard, but you can also create other subdomains if you wish.
Perhaps the most visible real world example of a site that uses subdomains is Google - which uses subdomains to manage each of its specific products within the site. If you take a look at the URLs when you are using Google products you will see examples like places.google.com, maps.google.com, blogger.google.com, webmasters.google.com, www.google.com etc.
A subdomain is referred to as a "third level" domain, created within another domain. As a rule, each level of a domain is defined by the dots (.) appearing in reverse order within a URL.
So, in the Google Maps example:
the Top Level Domain (TLD) is .com
the second level domain is the chosen domain name google
the third level domain (or subdomain) is maps
In your case it is a little more complicated. You have an extra level in your domains because your Top Level Domain is the country designator .uk so your second level is actually .co, but for the purposes of understanding subdomains it won't hurt for you to think of .co.uk as your top level domain.
Generally, most small sites don't require extra subdomains, so often the only subdomain in play is the www.
Matt Cutts has a simple tutorial on the parts of a URL that makes it much easier to understand what is going on at a glance.
Hope that helps,
Sha