This is an example of a virtual domain. In this case the good folks at example.com have registered example.net to prevent someone else from taking it and confusing their customers, but they want all inquiries to example.net to be directed to the same servers as example.com.

Lines 1 and 2 are NS records; they designate ns1.example.com. and ns2.example.com. as the name servers for example.net.

Line 3 designates mail.example.com as the mail server for example.net.

Line 4 is an A record, pointing inquires for example.net. to an IP address, which in this case that happens to be the same IP address as example.com.

Line 5 is a CNAME record telling the world to treat www.example.net. as if it were example.net.