Hey don't forget about UBEC's, or stand alone units.
Like was explained above, very well I must add, if the stock ESC is an Opto or not equipt with a built in BEC, or if the built in BEC is not able to handle the load, through too many servos or too high of a power feed, another option is a UBEC.

A built in BEC in a ESC, can only handle so much, amp and volts, otherwise all the magic smoke leaks out. If you are running more servos or higher voltage then the specs allow on the BEC, you must disconnect it. And use something in it's place. Disconnecting is as easy as removing the red wire from the ESC's plug that goes to the RX. Just pop it out of the plug and insulate it so it doesn't short against anything.

Now you can decide to either add a separate RX battery pack or a UBEC. A UBEC is wired to the main pack, most times with a switch, so it can shut off power to your system. It too has a plug that goes to any on the RX connectors, even Y connected to another lead.

Couple of benefits of a UBEC. Lot lighter weight then a separate RX pack, can handle higher voltage inputs and amp draws over the built in BEC, and in some cases is adjustable for voltage output. Like said above, 4.8 or 6 volts in most cases is the voltage used to power an RX/ servos. Jumping to 6 volts makes the servos work faster and makes them stronger.

That's why when you look at servo specs, you'll see both 4.8 and 6 volt listings of speed and output.

God I love electrics!