Heat rejection is the amount of heat energy that is transferred from the cool side to the warm side, plus the work done by the compressor. A cooling system transfers energy from the cool side to the warm side, along with the power fed to the compressor. 

Here are some heat rejection methods:
Air cooled: Air passes over a coil or channel containing fluid, transferring heat from the coil to the air.
Water cooled: A spray system passes water over coils or fill media to reject heat to the atmosphere through evaporation.
Rejecting heat into bodies of water: Allows for stable PUEs year-round.