A through-the-wall air conditioner is just what it sounds like. It goes straight through the wall. It's very similar to a window unit but is generally bigger and rather than sitting at the window, it's encased within a wall that faces the outdoors.
All these have lost some popularity as a means to cool a room, but they're still very much in use. Many companies depend on wall mounted air conditioner systems to maintain their spaces trendy. They're chosen over other kinds of units since they're semi-permanent but can be transferred from one place to another with minimal effort.
These kinds of units are common in temporary structures, such as those which are used on work sites. Typically these temporary buildings will have a cut in place for the air conditioner. The air purifier slides into the cut-out, the rear area of the unit hangs outside the building while the major part sits within the building.
In temporary constructions, the window frames might not be reinforced quite enough to hold the weight of a window air conditioner.
Installation is somewhat tricky, because a real hole in the wall needs to be made, and a casement needs to be set up. If you're extremely handy you may want to think about installing a through the wall air conditioner, if you have any bookings leave it up to the experts.