There are three classes of venting
a) direct vent
b) B-vent
c) no vent

In direct-vented products we use either co linear or
coaxial venting.  In either case one part of the system is
the exhaust and the other is the intake.  The unit itself is a
sealed combustion system and no air from the house is
involved.  The vents can go either through a sidewall or
through the roof. (see photo below)

In B-vent (natural vent) the units have a draft hood which
pulls air from the room to help dilute, thereby slowing, the
draft.  They also use room air for combustion.  They are
atmospherically vented with double-wall metal B-vent
pipe. B-vent is only vented vertically to the outside, but it
does not draw outside air for combustion. (See photo at
right).

No vent units (see photo bottom right) reflects the
ultimate in innovation and efficiency. Ideal for
contemporary living spaces, high rises, lofts or any space
where venting cannot be installed, the slim and modern
design makes installation as easy as hanging a picture on
a wall. They're less than eight inches deep! And because
this fireplace approaches a 100% fuel efficiency rate,
nearly all the heat produced is distributed directly into the
living space.