A vent free stove, fireplace or insert is strictly regulated to the amount of gas used in combustion.  Also the log set used cannot be altered or set up in any way other than what is specifically stated by the manufacturor.  This allows the unit to burn cleanly and efficiently.

These are for asthetics more than heat.  They have an opening ranging from 36" to 52" wide and 18" to 30" tall.  Glass doors are an option as well as a blower depending on the unit purchased.  Lennox HC42 - Superior BR36 - Heatilator ICON100 - Heat-N-Glo RH42 are some examples.

This type of wood burning fireplace is very efficient, just like a wood stove.  It is built to be used as a primary or secondary source of heat.  These units will heat usually from 1000 sq. ft. to 3500 sq. ft. depending on weather, insulation, heat distribution ect. and the size of unit purchased and installed.  Quadra-Fire FP7100 - Heat-N-Glo Northstar - Lennox Montecito - RSF Opel 3

A direct vent appliance exhausts through a small (usually 4 or 5" pipe) flue and draws combustion through a chamber created by the smaller pipe and a larger outer pipe (usually 7 or 8").  A direct vent appliance also has a fixed panel of glass on the front that must be kept on during operation.

An insert requires a preexisting masonry fireplace for its source of installation.  It is "inserted" into the existing fireplace.

A fireplace is any factory built appliance that can be installed into a wood framed structure.  The finishing material (i.e. stone, marble, tile, mantel) is then placed around it to finish the fireplace.  NOTE: There are circumstances in which steel studs are sometimes required.