A sub-domain is a domain that sits underneath the main domain name. It is anything before the ‘.’ before the main domain name. In the case of mtheory, mtheory.co.uk is the main domain name and tippingpoint.mtheory.co.uk is a sub-domain of mtheory.co.uk.
Sub-domains are useful as they can often be easily remembered and can redirect traffic somewhere else whilst appearing to stay within your domain. For instance tippingpoint.mtheory.co.uk is actually a Tumblr blog and mailboxes.mtheory.co.uk is actually hosted by our ISP, Dreamhost. They can also be used as a shortcut to some deeply nested part of your site that may otherwise require a very long URL.
If you need or want a sub-domain from mtheory for some particular use please get in touch with the administrator.