A Domain name has two parts, with the "top level" domain indicating the broad category of an organization.
For example, names ending in .COM are normally assigned to commercial, .NET normally for general web sites, names ending in .ORG are assigned to non-profit organizations. "Second level" domains represent your specific name, i.e.: yourname. Example being the "php-fusion" in our domain name would represent the second level portion of a domain name. The total length of the two-part domain name may be up to 67 characters (including the .co.uk).
The only characters allowed in a domain name are letters, digits (numerals) and the dash (-). A domain name can not begin or end with a dash. Capitalization (lower or upper cases) is ignored