OK, it's been covered but not always understood. What is a BEC? What is a UBEC? In an electric-powered radio controlled model, the BEC is typically part of the electronic speed control (ESC). BEC allows such a model to carry only one battery (the motive power battery) instead of two (motive power, and a separate battery to operate the R/C equipment). A BEC-equipped ESC meant for airplane use often incorporates a low-voltage-cutoff (LVC) circuit which can sense the voltage drop caused when the battery has little charge left in it. It then cuts the power to the 'drive' motor in order to provide the 'steering' servo(s) with enough power to be able to bring the model safely back to the operator. The power to the propeller would be cut but the operation of the control surfaces would be maintained in order to perform a dead-stick landing. Without this feature, all control would be lost when the battery expired, probably resulting in the destruction of the model. In some cases, the BEC is part of the radio control receiver, instead of being part of the ESC.R/C BECs in their simplest form use a linear fixed voltage regulator with its standard circuit suggested in the manufacturer's datasheet – usually the power supply of the receiver needs 5 V. Low-drop types are preferred – especially for batteries with only a few cells. For small models, 1.5 to 2 A are enough; for mid-size models a 3 A type needs to be considered. BECs for large models have to provide current of 5 A or more. There a more complicated switched-mode regulator should be used, as the BEC has to deal with losses. These losses are proportional to the difference of the target voltage of 5 volts and the voltage of the main battery; as well as they are proportional to the provided current. For example, take a 10-cell (NiMh) accumulator with a normal voltage of 12 volts. With a peak current of 5 A, the BEC will have losses of (12 V − 5 V) × 5 A = 35 W. With a linear regulator, these 35 W will be converted to heat and so require a large heat sink. In all cases, it is a good idea to mount some large capacitors to buffer the regulated output. In large plane or ship models, another possibility is to buffer the power supply with a further capacitor near the actuator's (servos). Hey, that was stolen from Wikipedia, no, I wrote the entry for Wikipedia.
Now, what's the difference between a BEC and a UBEC? These days, the term has become somewhat askew and considered the same and there are similarities. In the beginning of HV electricity in RC, capacitor based circuitry was used to supply the correct voltage to the receiver and thusly the components. Along came the UBEC Universal Battery Eliminator Circuit. They were big, clunky and always external. And then along comes the ESC with the appropriate circuitry built in and the common name BEC was used.
So, what's the difference. Again, the terms are askew no days but the original nomenclature of each is this. The BEC is linear while the UBEC is considered a switch mode BEC. The BEC has a more undisturbed signal an voltage and cheaper to manufacture. Adversely, they are less efficient and generate higher temperatures at greater voltages. The UBEC generally handles higher voltages, runs cooler and is more efficient. On the down side, they have an increased component count so they are more expensive to produce and they have somewhat of a dull hum that makes them louder.
Here's a modified pic to show you how to hook one up.