This tool allows you to query servers of the worldwide Domain Name System (DNS). A DNS record consists of SOA (start of authority) records, NS (name server) records, A (address) records, CNAME (canonical name) records, MX (mail exchange) records and a few more. Those records are all saved in the zone file, which contains all the name server indications of a domain name.
More information
Address (A)
A records allow you to point a name to the desired IP address. Every domain name should dispose of a primary A record, which assigns the domain to an IP address. With this, it is possible to assign subdomains to a completely different server.
CNAME (canonical name record)
A CNAME record lets you create an alias for an existing A record. A CNAME record always points to an existing A record and returns the value of the referenced A record when queried. It is also possible to define an alias to a canonical name on another domain.
Mail Exchange (MX)
The MX record defines the mailserver ("mail exchanger") of a domain. MX records can only point to an already existing record just like CNAME records. MX records differ from CNAME records by having a third parameter, which defines the preference. The server sending the mails tries to deliver the e-mails in order of this preference-value (ascending). The preference can be defined by using values from 0 to 65535.
NS (name server)
A NS record defines which name server are officially responsible for this zone. At least one NS record has to be present which indicates the authoritative name server for this zone.
PTR (pointer)
PTR records are the counterpart to a classic allocation of one or more IP addresses to a given host name via A record.
TXT (text)
A TXT record can be used to deposit a freely definable text in a DNS zone. This record is also used for SPF records. Though, dedicated SPF records exist by now for exactly this purpose.
HINFO (host info)
The HINFO record describes the hardware and the used operating system.
You can find further information regarding DNS records over here.