A Records in DNS - What are...

A Records in DNS point Subdomains to IP Addresses. So MAIL.domain.com points to 10.1.10.100, and VPN.domain.com points to 10.1.10.101.
Generally an A Record is created for @ that points to your web server. Then in C Names you point WWW to @ using @ as a variable. You can point WWW directly to an IP address in an A Record but it is not a "best practice". The @ allows you to point multiple CNames to the main server, and then only change the IP address of the main server when needed. So in CNames you would point WWW to @, FTP to @, and VPN to @ if those services all reside on the main server.