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

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.

DHCID : DHCP identifier (RFC 4701)
Used in conjunction with the FQDN option to DHCP

RRSIG : DNSSEC signature (RFC 4034)
Signature for a DNSSEC-secured record set. Uses the same format as the SIG record.

TSIG : Transaction Signature (RFC 2845)
Record that supports one set of security mechanisms for DNS. Used to secure communication between DNS resolvers and Name servers, in contrast to DNSSEC, which secures the actual DNS records from the authoritative name server.

TA : DNSSEC Trust Authorities (None)
Part of a deployment proposal for DNSSEC without a signed DNS root. See the IANA database and Weiler Spec] for details. Uses the same format as the DS record.

AFSDB : AFS database record (RFC 1183)
Location of database servers of an AFS cell. This record is commonly used by AFS clients to contact AFS cells outside their local domain. A subtype of this record is used by the obsolete DCE/DFS file system.

SRV : Service locator (RFC 2782)
Generalized service location record, used for newer protocols instead of creating protocol-specific records such as MX.

SPF : SPF record (RFC 4408)
Specified as part of the SPF protocol, as an alternative to storing SPF data in TXT records. Uses the same format as the TXT record.

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.

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.

DNAME : delegation name (RFC 2672)
DNAME will delegate an entire portion of the DNS tree under a new name. In contrast, the CNAME record creates an alias of a single name. Like the CNAME record, the DNS lookup will continue by retrying the lookup with the new name.

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.

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

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

NAPTR : Naming Authority Pointer (RFC 3403)
Allows regular expression based rewriting of domain names which can then be used as URIs, further domain names to lookups, etc.

DS : Delegation signer (RFC 4034)
The record used to identify the DNSSEC signing key of a delegated zone

LOC : Location record (RFC 1876)
Specifies a geographical location associated with a domain name