Hey guys, here is a list with on pmc used abbreviations and orther therms.

LAN= Local Area Network (the mod is playable in lan)

WIP= Work in Progress (the mod/build is not finished yet)

FML= Forge Mod Loader (this mod requires forge)

HG/SG= Hunger games/ Survival games

BUKKIT (CraftBukkit-server)= this is a bukkit server

BUKKIT = this is a bukkit plugin, NOT a mod

Discontinued= the mod author stops updating the mod.

SSP= SurvivalSinglePlayer

SMP= SurvivalMultiPlayer

Vannilla Minecraft=  Default Minecraft. Even if you have modded, you still have vanilla things in Minecraft. For instance, the /help command is avaliable in vanilla Minecraft, but it's also a part of bukkit. The commands does the same thing, it's just that there's "vanilla help" and "bukkit help". 
Vanilla Minecraft is always with you, even how hardcore you've modded your client. Vanilla is always down in the bottom under all your mods

Pop Reel= the post as been on the homepage for a while

NMS= The core code of Minecraft

MOTD= Message Of The Day

PVP= Player Vs Player (fighting)

---In Game---
gg= Good Game

kk= ok

k= ok

gl= Good Luck

hf= Have Fun

ikr= I Know Right

gtg= Got To Go

brb= Be RIght Back

afk= Away From Keyboard

wp= Well Played

mlg= Major League Gaming

ftw= For The Win