CNAME-records are domain name aliases. Often computers on the Internet have multiple functions such as web-server, ftp-server, chat-server etc. To mask this, CNAME-records can be used to give a single computer multiple names (aliases).
For example computer "xyz.com" may be both a web-server and an ftp-server, so two CNAME-records are defined: "www.xyz.com" = "xyz.com" and "ftp.xyz.com" = "xyz.com". Sometimes a single server computer hosts many different domain names (take ISPs), and so CNAME-records may be defined such as "www.abc.com" = "www.xyz.com". A CNAME-record should always point to an A-record to avoid circular references.