A : address record (RFC 1035)
Returns a 32-bit IPv4 address, most commonly used to map hostnames to an IP address of the host, but also used for DNSBLs, storing subnet masks in RFC 1101, etc.

NS : name server record (RFC 1035)
Delegates a DNS zone to use the given authoritative name servers

DNSKEY : DNS Key record (RFC 4034)
The key record used in DNSSEC. Uses the same format as the KEY record.

PTR : pointer record (RFC 1035)
Pointer to a canonical name. Unlike a CNAME, DNS processing does NOT proceed, just the name is returned. The most common use is for implementing reverse DNS lookups, but other uses include such things as DNS-SD.

SOA : start of authority record (RFC 1035)
Specifies authoritative information about a DNS zone, including the primary name server, the email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone.

CNAME : Canonical name record (RFC 1035)
Alias of one name to another: the DNS lookup will continue by retrying the lookup with the new name.

MX : mail exchange record (RFC 1035)
Maps a domain name to a list of mail exchange servers for that domain

NSEC3 : NSEC record version 3 (RFC 5155)
An extension to DNSSEC that allows proof of nonexistence for a name without permitting zonewalking

OPT : Option (RFC 2671)
This is a 'pseudo DNS record type' needed to support EDNS

AAAA : IPv6 address record (RFC 3596)
Returns a 128-bit IPv6 address, most commonly used to map hostnames to an IP address of the host.

TXT : Text record (RFC 1035)
Originally for arbitrary human-readable text in a DNS record. Since the early 1990s, however, this record more often carries machine-readable data, such as specified by RFC 1464, opportunistic encryption, Sender Policy Framework, DomainKeys, DNS-SD, etc.

IXFR : Incremental Zone Transfer (RFC 1995)
Requests a zone transfer of the given zone but only differences from a previous serial number. This request may be ignored and a full (AXFR) sent in response if the authoritative server is unable to fulfill the request due to configuration or lack of required deltas.

HIP : Host Identity Protocol (RFC 5205)
Method of separating the end-point identifier and locator roles of IP addresses.

TKEY : Transaction Key (RFC 2930)
One way of providing a key to be used with TSIG

NSEC : Next-Secure record (RFC 4034)
Part of DNSSEC—used to prove a name does not exist. Uses the same format as the (obsolete) NXT record.

KEY : Key record (RFC 4034)
Used only for TKEY (RFC 2930). Before RFC 3755 was published, this was also used for DNSSEC, but DNSSEC now uses DNSKEY.

DLV : DNSSEC Lookaside Validation record (RFC 4431)
For publishing DNSSEC trust anchors outside of the DNS delegation chain. Uses the same format as the DS record. RFC 5074 describes a way of using these records.

AXFR : Full Zone Transfer (RFC 1035)
Transfer entire zone file from the master name server to secondary name servers.