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.

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.

IPSECKEY : IPSEC Key (RFC 4025)
Key record that can be used with IPSEC

SIG : Signature (RFC 2535)
Signature record used in SIG(0) (RFC 2931). Until RFC 3755 was published, the SIG record was part of DNSSEC; now RRSIG is used for that.

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

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

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.

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

SSHFP : SSH Public Key Fingerprint (RFC 4255)
Resource record for publishing SSH public host key fingerprints in the DNS System, in order to aid in verifying the authenticity of the host.

NSEC3PARAM : NSEC3 parameters (RFC 5155)
Parameter record for use with NSEC3

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

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

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.

CERT : Certificate record (RFC 4398)
Stores PKIX, SPKI, PGP, etc.

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.

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.

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