What is a CNAME Record?
A CNAME Record (Canonical Name Record) is a type of DNS Record that shows one domain as being an alias for another domain - the so-called canonical domain. For example, if you wish to redirect foo.example.com to example.com, then you would direct the CNAME Record for foo.example.com to the A Record for example.com. Then if you ever change your IP address, you only need to change the A Record for example.com.
Note that CNAME Records must point only at a domain name, not at an IP address.
You should avoid pointing one domain with a CNAME Record at another domain name with a CNAME Record, as this is inefficient. Instead you should point both domains at the same A Record.
You should not point an MX Record at a CNAME alias.
CNAME aliases should not have other DNS Records. The only exceptions to this are certain DNSSEC Records.