Most esc's have a Battery Eliminating Circuit in them that takes your main battery voltage and drops it down to 5 or 6v for your servos and receiver.
The type of bec's most controllers have in them are not very efficient and create heat. The higher your battery voltage the more heat they will create. They shouldn't create enough heat by themselves to make the esc very hot, but when the esc is already getting hot from powering the motor, a hot bec just adds to the problem.
Since the internal ones create more heat in the controller and can't be used with higher voltages, most of us use External BEC's. Because they work differently to most of the internal ones, they are more efficient, usually put out more power to the servos, and any heat they do create (they create less heat than the internal ones, because of the way they work) doesn't end up in your esc. The UBEC and CCBEC are just two of the different versions of external bec's. Using a rx pack will do the same thing, but most like the ebec's, because they are lighter and you don't have to recharge them.
Some of the new MGM's, like the 16018 have built in switching bec's (the same as the external ones) so you don't need to have an external bec.
UBEC (the one I'm using)
http://www.rc-monster.com/proddetail...=6vubec&cat=21
CCBEC
http://www.rc-monster.com/proddetail...stlebec&cat=21
I think the CCBEC is set to 5v from the factory, so if you want to change the voltage to 6v or whatever to get the most from your servos, you have to use the programing cable
http://www.rc-monster.com/proddetail...phx-lnk&cat=21