If you have domain-a.com you can register domain-b.com and add a domain alias for domain-b.com. Than http://domain-a.com and http://domain-b.com would take a visitor to the exact same pages of your domain-a.com website. www.domain-a.com mail.domain-a.com whatever.domain-a.com in these examples www, mail and whatever are subdomains and can take a visitor to same or different location as domain-a.com .com is a top level domain .domain-a is a second level domain or a subdomain of domain .com .whatever is a third level domain or a subdomain of domain-a.com