The ductless mini-split system is a hybrid between central systems and window units. They are split systems in that the cooling coil is inside and the compressor, condenser coil and fan are located outside. However, no ductwork is needed and the typical system needs just a 3" hole through the side of your home to install the system.
The inside unit (or cassette, as it is sometimes called) is generally a long, rectangular unit that mounts above a window or high on a wall. It can be individually controlled by a remote or thermostat and many mini-split systems can support up to four of these inside units.
They share all the benefits of window/room units with the addition that certain models can also provide heat in cold months. Their main disadvantage is their cost relative to less sophisticated window alternatives. Using a qualified contractor for these units is especially important as they are a relatively new technology in the United States.