A window mounted air conditioner is meant to be installed through a window and has side extension to fill any gap that that may be left open. A wall mounted air conditioner is made to be installed into a opening that has been cut into a wall. A window unit does not require any major home modifications. All that is needed is the correct window dimension to be known so that the correct size unit can be bought. Window units have the disadvantage of never completely having the area on the sides properly sealed and insulated. Which can leak out the cold air a A/C unit is pumping in. Window units also block the use of the window it is placed in. Wall mounted units have a better seal around them since a hole is cut into a wall for the unit to be placed in. This makes them more efficient to use. Wall units also don’t block a window from being used and look more professional. Window A/C units are much easier to install since no hole needs to be cut into a wall.