peltier cooling uses thermoelectric cooling modules to remove heat from the CPU via the peltier effect. when you run electricity thru a peltier, one side will get hot and the other will get cold - they pump heat from one side to the other. you have to add another method to cool the hot side or they will burn out. usually, a peltier is mounted like so:
----------cpu-----------
----copper cold plate----
---------peltier---------
------water block--------
good info on the basics of how peltiers work can be found here:
http://www.electronics-cooling.com/Resources/EC_Articles/SEP96/sep96_04.htm
phase change - in the extreme cooling section, this is usually talking about cooling systems that are very much like the one in your air conditioner or refrigerator. you have a compressor, a condenser, a metering device and an evaporator. The loop is filled with a refrigerant like R134, R22, R404a, R507, etc.
Depending on the type of system, the evaporator can be submerged in coolant to get the coolant temps subambient and then a fairly standard but well insulated watercooling loop moves the chilled coolant to waterblocks on various components in the system to remove heat. this is a waterchiller system.
In a direct die phase change system, the evaporator is usually a small block of copper (2 x 2" or so) with a maze type pattern cut in the internals of the block. refrigerant is fed into the evaporator to remove heat - the evaporator is mounted to the CPU in a way much like a waterblock would be mounted to a CPU (once again, with insulation). Since these systems can remove lots of heat, they can supercool the CPU (how cold they get depends on the capacity of the phase change system).
for much more info on how this works:
http://oberon.ark.com/~airekool/rb1.htm
a great example of a direct die evaporator is the baker evaporator:
http://www.xtwerkz.com/baker_cpu_blocks.htm
normally, colder is better. the reason some try to stay at or near ambient temps is they don't want to have a nice battle with condensation. pelts or phase change evaps get far below ambient temps - hence, condensation will form on them unless they are heavily insulated. If you get condensation, you will eventually collect enough moisture on the surface of your cooling solution as to where you will get water on your components (it will collect and then run onto the CPU, mobo, etc.) So, you can either stay near ambient or resort to heavy anti-condensation tactics.
that was a really basic explanation but the above links will shed more light on the "how it works" part.
awsome post weapon!! great links too!
