Window units, or the larger version known as through-the-wall units, are the smallest of the self contained machines. Inside of the cabinet is a compressor, condensing coil, evaporator coil, blower, controls, and maybe a little ductwork. The ductwork and possibly a damper will allow the unit to draw fresh air in or re-circulate 100% of room air. The high end units can have electronics with timers, accurate temperature sensors, and remote control. By using sound absorbing materials and quieter motors, manufacturers have improved these machines over the years.
The efficiency ratings are not as high as central systems (EER ratings) and versatility is limited because all of the cooling is blown into one room, but they get the job done when needed.
Not obvious is the fact that they need maintenance routinely, just like any other machine. The filter, usually located behind the front grill, needs to be washed. The condenser coil at the back of the unit can accumulate a lot of debris, and might require a special chemical bath. All air conditioning systems, large or small, need good air flow across the evaporator coil and the condenser coil. Efficiency and performance drop rapidly as air flow decreases. Lose enough air flow and the unit will burn out. Rodents can nest inside when not in use, and can foul up the unit in many ways. Wasps and hornets, as well as the honey bee find finned coils to be comfortable nesting places. Nests and dead insects can ruin the air conditioner.