The most common air conditioning units we see nowadays is air-cooled split unit. It used to be window unit many years ago. I guess it must be quietness of split units that won hearts of users.
In any air conditioning unit, there must be a process for absorbing heat from indoors and discharging it to outdoors. For air-cooled units, heat is discharged directly to atmosphere.
What's difference then between window units and split units?
In window units, whole refrigeration circuit is installed in a box-like enclosure. The cold portion is separated from hot portion by partitions. As name suggests, usually it is installed at a window, mainly because this is easiest place to make an opening in wall. Failure to find a window opening may mean hacking wall to create an opening roughly same size as window air cond unit.
Because compressor, blower fan, condenser fans and other moving machinery are all installed in same box, much of noise they make gets transmitted to room indoors. This type of air conditioning unit tends to be quite noisy.