Range hoods are the decorative covers for kitchen exhaust fans. Exhaust fans are needed to provide ventilation for cooking appliances, primarily the cook top. They are especially important for gas appliances where combustion by-products also need to be removed from the home.
Some range hoods just filter the air, recirculating it back into the home. Ones that exhaust the air to the outside are better. To tell what type you have, look in the cabinet about the range hood and see if there is a vent pipe. If that area is inaccessible, run the fan and see if air blows out the front of the unit. If it does, the unit is not vented to the outside. The only advantage of a recycling vent hood is that you won't have air leaks around the roof or wall penetration that are notorious for allowing outside air to leak into the home. If you have a vented unit, check to see if the area where it passes through the wall or ceiling is well sealed. And be sure to keep it off when it's not needed. Running it when the heating or cooling system is operating pulls expensive conditioned air from your home and dumps it to the outdoors.
While it's not something you'd likely change in your home unless you are remodeling, if you are replacing a range hood, be sure to get a properly sized unit. Undersized units won't adequately ventilate, and oversized ones waste your conditioned air when the heating or cooling system is operating.