Gateway: A device that translates SIP into some other realtime communication protocol. Usually, this is a SIP-to-ISUP gateway used to connect SIP networks to the PSTN.

IETF (Internet Engineering Task Force): The open-standards body tasked with developing and maintaining Internet protocols.
IAD (Integrated Access Device): An IAD is a device that multiplexes a variety of communications technologies in the customer’s premises onto a single telephone line for transmission to the carrier. An IAD is always necessary as a gateway to an analog PBX and Key Set phone system, while IP-PBX may support the native interface of a SIP Trunk.
RTP (Realtime Transport Protocol): The protocol used to carry media between endpoints. RTP packets contain a time stamp, some sequence data, and a chunk of encoded media. RTP receivers use the information to reconstruct the media stream, accurately accounting for packet delay, jitter, and loss.

SIP (Session Initiation Protocol): An IETF protocol used to establish realtime communications sessions. SIP allows endpoints wish to communicate to find each other and negotiate how they want to exchange media.
SIP URI: A uniform resource identifier with the scheme “sip:”. An example is sip:RjS@example.net. SIP URIs identify a particular resource at a domain (example.net). SIP systems use the domain component along with Domain Name Server (DNS) to determine where to send SIP messages.

Transport protocol: The underlying Internet protocol used to carry SIP or RTP messages. Defined transport protocols for SIP include UDP, TCP, TLS, and SCTP. RTP is carried over UDP. Work is under way for both protocols to define transporting them over DTLS.