Guide to DNS
Sub-domain (also called third level domains) which is technically called canonical domains (or CNAME) for short. The sub-domains are like having an additional domain name and can be almost anything you like. In a domain name as subdomain.domain.com, subdomain is the subdomain. It works the same as a regular domain name.
A Record of Directors (or record) is the basic DNS record and more important. The records point to an IP address. His short domain name (without www), NS, and FTP should have the records. Subdomains sometimes have a Record as well. The records can point to any IP address.
CNAME-Records (or canonical domains) are sub-domains and aliases used CNAME to point to a domain name or a file on a domain. However, CNAME must always point to an A-remember, not another CNAME. It is common practice to create a CNAME for www and any subdomains that actually hosts your domain. CNAC also be used as temporary alias to point your domain to another domain.
Note: when you point a CNAME, always put a period after the domain (ie. FTP -> CNAME -> domain.com)