What are CNAME records?
CNAME records are aliases of, or redirect to, other DNS entries.
If domain.com points to 184.108.40.206 and you set up www.domain.com as a CNAME of domain.com - www.domain.com will now also point to 220.127.116.11
When are CNAME records used?
Most commonly, CNAME records are used to map the www. version of a domain to the root domain. For example, you would use a CNAME record to map www.domain.com to domain.com so when people visit www.domain.com they are pointed to domain.com.
In the above example, the www. and mail. entries are CNAME records of dummyaccount.co.uk.
Why use CNAME records?
It is quite common to find that everything you have related to a domain is hosted in the same place - The website, mail, localhost, webmail etc will all use the same IP.
You could set up a specific record for each of these things, but if they ever changed every single one would need updating separately. With CNAME's you would only need to update the original record as the rest would draw their information from that one record.