There are far too many of the damned things. Let's try and make this comprehensive. Please try and keep in alphabetical order. Descriptions should be short.

Acronym Meaning Description
ACK Acknowledge Packet type in TCP that lets the transmitter know a data packet has been successfully received.
ARP Address Resolution Protocol Protocol used to detirmine the link layer address associated with a network layer address.
AS Autonomous System A large routing region for the BGP. Specified using an AS number.
BGP Border Gateway Protocol Application layer protocol used for interdomain/exterior routing
CIDR Classless Inter-Domain Routing The new method of specifying IP subnets by the number of bits in the network portion of the address, e.g.
CRC Cyclic Redundancy Checksum Algorithm specified by TCP/IP for calculating a packet checksum.
CSMA Carrier Sense Multiple Access MAC protocol used by ethernet. Sometimes 'CSMA/CD', the CD at the end means 'with Collision Detection'
DHCP Dynamic Host Configuration Protocol Application layer protocol for providing a newly-connected client with information about the local network.
DNS Domain Name Service Application layer protocol that obtains information (most importantly IP address) associated with a domain name.
FTTN Fibre To The Node Refers to a physical network architecture where optic fibre runs all the way to the local exchanged.
FTTH/FTTP Fibre To The Home/Premises Refers to a physical network architecture where optic fibre runs all the way to your house or property, for the ultimate internet connection.
GPON Gigabit Passive Optical Network FTTP standard specifying physical and link layers for ~2Gbs downstream and ~1Gbs upstream
HTTP Hyper Text Transport Protocol Application layer protocol for sending html and other MIME types to web browsers.
ICMP Internet Control Message Protocol Network layer protocol used for network management and configuration.
IGMP Internet Group Management Protocol Network layer protocol used for managing multicast groups in IPv4.
IGP Interior Gateway Protocol Application layer protocol for interior routing
IMAP Internet Message Access Protocol Email protocol. Facilitates remote access to a mail account.
IP Internet Protocol the network layer protocol.
LAN Local Area Network Pretty self explanatory. Technically, a bunch of nodes that can reach each other directly by link layer.
LSA Link State Advertisement A distributed routing technique used to calculate shortest paths.
MAC Media Access Control OSI protocol layer that is part of the link layer. Examples are CSMA/CD (Ethernet) and Aloha.
MEGACO Media Gateway Control Application layer protocol for controlling media gateways. Also called referred to as H.248
MTA Message Transfer Agent Entity in SMTP that acts as both a server and a client.
MTU Maximum Transmission Unit The largest possible packet size for a protocol. Bigger is generally better.
NAT Network Address Translation Dirty hack for presenting an entire network to the rest of the world behind one IP address, by mangling packet IP addresses on the fly.
NE Network Element Generic name for a piece of equipment or node in a network.
OSPF Open Shortest Path First A network layer protocol that uses LSA for distributed routing.
POP Post Office Protocol Email protocol. Fetches email from a remote server.
PPP Point-to-Point Protocol Link layer protocol used for serial connections between two directly connected machines.
RFC Request For Comment Standards document describing an Internet protocol or technology.
RIP Routing Information Protocol An application layer protocol that uses distance-vector algorithm for distributed routing.
RST Reset Packet type in TCP used to end a connection.
RTP Realtime Transport Protocol Application layer protocol for transporting audio and video, usually over UDP. Emphasizes low latency over reliability.
RTT Round Trip Time In TCP, used to determine how long to wait for and ACK before retransmitting.
RX Receive(d) A contraction, often used in the context of wireless link layers.
SCTP Stream Control Transfer Protocol A new-and-improved transport layer protocol with features from both TCP and UDP along with fancy stuff like multihoming. Expected to be widely adopted shortly after IPv6/hell freezes over.
SMTP Simple Mail Transfer Protocol Email protocol. Used for sending email.
SSL Secure Sockets Layer see TLS
SYN Synchronise Packet type in TCP that is used to initiate a connection.
TCP Transmission Control Protocol The most commonly used transport layer protocol. Provides reliable connections over IP.
TLS Transport Layer Security Application layer protocol that wraps a transport layer protocol to provide cryptographic authentication and security to another application layer protocol such as HTTP.
TX Transmit(ed) A contraction, often used in the context of wireless link layers.
UDP User Datagram Protocol The second most commonly used transport layer protocol. Flings packets at you with no guarantee of receipt.
WAN Wide Area Network The rest of the world that isn't on the LAN.