Not all portable air con units need emptying, some have a system to evaporate the condensate in the exhaust.
With regard to how they work, they compress a gas which makes it very cold, then move that gas through a coil. They then force the air in the room over that coil to cool it. The problem is that the compressor itself produces heat, but at least that is easy to manage and is pushed out of the room. As mentioned on another thread, if you have a single-hose system, the slight drawback is that air will be pulled into the room to replace that which is pushed out.