A domain name is this bit:
http://www.pollyprojects.co.uk
Not the
http://www.
Just the
pollyprojects.co.uk
pollyprojects is the unique part of the domain name, which you can have as whatever you'd like (as long as it's available for purchase - check here)
.co.uk is the suffix. The suffix will be from a list of preset endings, e.g. you can have .com or .tv or .info. Most suffixes are priced slightly differently (see here for a more detailed list of costs).