If you're deciding on a kitchen exhaust fan, there are two air-flow options: updraft and downdraft. These determine how cooking vapors (steam, smoke, etc.) are expelled from your home.
The most common kitchen exhaust option is updraft. It is installed into overhead ventilation systems called range hoods. They remove cooking fumes by pulling them up with a motorized fan. The fan then exits the gases via a duct connected to the outside of the house.
Downdraft exhaust units pull air down instead of up. This option is preferred when space constraints exist above the stove. Fumes are vented down to the cooking surface by a fan located there. The fan exits the fumes through ductwork located at the back of the base cabinet or under the floor.
A downdraft exhaust has greater limitations than its updraft counterpart. The fan must be installed in the base cabinet, which leads to a loss of space. Also, downdraft exhausts are only available on drop-in cooktops. Given ductwork has to pass through the floor, venting may be limited. If a concrete slab lies underneath, ductwork has to be installed before the slab is poured. Downdraft exhausts also require more flat counter space.