At work I tend to be technical support to a lot of clients (though it is not my job), one of the main jobs is handling DNS entries. DNS or the Domain Name System is the structure for naming conventions for computers. Since everything is addressed through IP addresses for every computer location it is important that we humans have some sort of understandable form in which the normal user can understand.
Anyway during my work as a web developer I find it really important to understand the terminology on managing websites especially if you run your own server. This covers the importance of A records, MX records, sub domain structure and zones.
Wikipedia is an excellent source of information regarding the DNS structure. which I advise any freelancer to look up because client will always come to you regarding domain transfers, emails not working and in a lot of cases a domain name is registered with someone completely different to the hosting.