Folks on Hearth.com often use shorthand for their forum posts.

Below are some of the common abbreviations and other confusing terms you might find:

OAK: Outside Air Kit
PE: Pacific Energy
VC: Vermont Castings
DW: Dutch West
CB: Central Boiler
NG: Natural Gas
SS: Stainless Steel
ZC: Zero Clearance
HT: High Temperature (also used to describe a class of chimney)
FG: Fiberglass Insuation
EPA: Environmental Protection Agency (USA- makes standards for clean burning)
DHW: Domestic Hot Water
OWB: Outside Wood Boiler or Furnace
CFM: Cubic Foot per Minute – usually of a fan
NFPA: National Fire Protection Association (sets fire standards)
UL: Underwriters Laboratories (sets and tests to standards)

Top down: method of starting a fire, smallest on top to larger on bottom
Holz Hausen: A method of stacking wood in a circle with air space in the center

(note: please add your own others!)

Note: here is a full glossary of Hearth Related Terms:
http://www.hearth.com/econtent/index.php/articles/glossary