CNAME is canonical name record. Such record allows you to map more than one name to the same computer and it is generally applying to the computers offering web and email services on the same one. For example, there is a computer named 'host.mydomain.com' (A record) offering both www and mail services. For the users' convenience to access the websites, you can give two CNAME to the computer: www and mail. The full names of these two aliases are www.mydomain.com and mail.mydomain.com, actually both of them are pointing to host.mydomain.com.