A sub domain site is a website that is created on as part of the same domain name as a current site. In the eyes of the search engines, a sub domain website is a totally standalone site. A normal domain would be example.co.uk where as a sub domain would be xxx.example.co.uk with the xxx’s being replaced with the name of the site.

So when would you need a subdomain? If you have a large amount of content on your main website that is cluttering it up you may want to move it to a sub domain and create a mini website. This site will need to be SEO’ed as with the main site but once live and indexed you can actually gain better results by having a site that is completely tailored to a specific subject. You can also link this site to your main site in order to create a backlink.