ADB - Android Debugger, a utility to run on a PC to allow connection to and control of an Android device
AKU - Adaptation Kit Update: they usually patch up existing bugs and enable several new features. Each newly released AKU pack retains fixes found in previous versions of AKU
CSC - File with Carrier Customizations
CID lock (aka vendor lock) - put on your device by the manufacturer to prevent installation of a ROM not released by them
CM - CyanogenMod, an alternate firmware for your phone, with the stuff your carrier left out, and more
CWM - ClockWorkRecoveryMod, an alternate recovery mode for your phone, with the stuff your carrier left out, and more
ESN - Electronic Serial Number unique to your phone (usually in hexadecimal or decimal)
FC - Force Close, AKA application crashed
gapps - Google Applications, like GMail, Calendar, Maps, G+, etc
GB - Ginger Bread, AKA Android OS 3
ICS - Ice Cream Sandwich, AKA Android OS 4
IPL - Initial Program Loader: Its the bootloader for PPC. It boots up SPL.
MMS - Multimedia Messaging Service (text messaging with pictures)
MSL - Master Security Lock (the six digit code needed to program your phone)
OCUV - Over Clock/Under Volt the CPU to save power and increase speed
OTA - Over-the-Air (usually used as "OTA update" for firmware updates that can be installed directly via 3G/4G/Wi-Fi without the need of an PC)
PRI - Product Release Instructions (default settings your phone ships with)
PRL - Preferred Roaming List (tells your cellphone whose towers to use when roaming)
RIL - Radio Interface Layer
ROM - Read-only memory. Can also refer to a custom ROM firmware file
RUU - ROM Upgrade Utility: Its the s/w used on your PC to do a ROM upgrade for your device
SMS - Short Messaging Service (text messaging)
SPL - Secondary Program Loader: Loads the actual OS.
WiMAX - Worldwide Interoperability for Microwave Access.
WWE Edition - World Wide English Edition
UC - User Customization
XIP - Execute-in-Place
More in the FAQ Section.