Resource Records

Inside the zones we define the Resource Records (RR), these are defined in the RFCs 1035 and 3596, which describe each of the elements that make the zone. Each RR has the following information:

The domain that owns the RR.
The type of register contained in the RR. The most common RR are:
It stores an IP address (IPv4), associated to a name.
It stores an IP address (IPv6), associated to a name.
Canonical Name, stores an alias to another domain name, it is not allowed that an alias points to another one, only one level of indirection is allowed.
Mail eXchanger, it holds the host name that hosts the mail exchanger for the domain, it must be an A RR, CNAMES are not allowed as only one level of indirection is allowed, a zone can hold several MX records with different priorities, lower numbers indicate higher priorities. This record is used by mail servers (SMTP) to find the mail exchanger for the mail sent to that domain and it's a basic element of the mail systems.
Name Server, it holds the an authoritative name server for the domain, these servers must be host names, and, as with MX records, they must be A records. NS records also allow to set delegations of a part of the tree to other servers, these delegations constitute new zones under a different administrative authority.
Domain pointer, generally use together with the domain in-addr.arp to implement reverse resolution: converting IP addresses into domain names.
Start Of Authority, defines the start of a DNS zone, contains information about it: the server that holds it, administrator e-mail address, serial number, default TTL, expiry, etc...
The protocol or family of protocols the record belongs to, even though several classes exist the only one commonly used is IN that refers to Internet addresses, the others are CH (Chaos) and HS (Hesiod)
Time To Live, it's the period of time an RR record can be cached by a server that does recursive resolution.
The information a RR register holds, this is dependent upon the kind of RR and it can be an IP address, a domain name, a host name, etc...

RR examples (in the format used in zone master files):

;Owner                 TTL     Class  Type    RDATA        172800  IN     A
                        172800  IN     A            172800  IN     NS

If, like in the example above, a register does not have an owner, the owner is the owner of the previous register.