Learn about voltage regulation: Supplying your electrical components with the correct voltage is crucial for their operation. LiPo battery cells are rated at 3.7 volts per cell, therefore a range of voltages are available, leading from 3.7v (1S) to 44.4v (12S). A 3 cell 11.1v pack is widely employed as it provides a handy voltage for many electronics without the need to regulate. However, more powerful multirotors require LiPos with greater voltages; 6S (22.2v) is common for octocopters, thus voltage regulation is necessary to supply power to auxiliary components, such as the: receiver, flight controller, camera, transmitter and lights etc.
There are two types of regulators, linear and switching. A linear regulator works by taking the difference between the input and output voltages, and just burning it up as waste heat. The larger the difference between the input and output voltage, the more heat produced. This is the reason why linear regulators are usually only compatible with up to 3S LiPos. Linear regulators typically operate with efficiencies of 40%, reaching as low as 14%. On the other hand, switching regulators are far more efficient. They work by storing pulses of energy taken from the battery in a capacitor and then releasing it. This is accomplished with the help of an electrical switch and a controller, which regulates the rate at which energy is transferred to the output (hence the term “switching regulator”). The energy losses involved in moving chunks of energy around in this way are relatively small, and the result is that a switching regulator can typically have 85% efficiency. Since their efficiency is less dependent on input voltage, they are compatible with higher voltage LiPos. While they are preferable to linear regulators you should consider their placement within the UAV carefully, as in some cases the switching noise can cause interference with the receiver.
Within a multirotor context, the term "BEC" (Battery Eliminator Circuit) is exchangeable with voltage regulator. The use of a BEC negates the traditional need for a separate battery to power you flight auxiliaries, for example a receiver. Most speed controllers (Not OPTO ESCs) incorporate an internal 5v linear BEC, so that simply plugging the ESC into the receiver is all you need to supply power to it. However, as mentioned above they only work well under low loads and at low voltages. For higher voltage/current applications it is recommended to use a stand-alone SBEC - that way you reduce the risk of a brownout (a term used to describe the temporary loss of function of the radio receiver due to the supply voltage dropping too low). Confusion often surrounds the difference between a UBEC and an SBEC, however they are both the same thing - a switching voltage regulator. "UBEC" was once a specific brand name for one of the original switching BECs, so the more correct generic term is actually SBEC.