The circuits inside robots often consist of modules with different DC supply voltage requirements. Microcontroller boards like the Arduino, single-board computers like the Raspberry Pi, and logic circuitry usually require 5 V or 3.3 V supply voltage and can draw currents up to 2 A. Actuators and sensors, on the other hand, can operate on a wide range of voltages and currents. The power requirements of the Robofish robots are comparable to those of radio-controlled toy vehicles. I use high-performance lithium polymer (LiPo) batteries that are rated at 7.4 V or 11.1 V and can provide ample current to the DC motors. The Arduino has a built-in regulator with a wide input range from 6 V and 20 V, although it is recommended to stick to voltages between 7 V and 12 V. For the Raspberry Pi, however, the battery voltage needs to be stepped down to 5V. That's what voltage regulators are for. And there are two kinds that you will care about: linear regulators and switching regulators.
Linear Regulators
Linear regulators have been around for some time and are cheap to manufacture. Input and output currents are more or less the same. The voltage drop leaves energy behind in the regulator, which is dissipated thermally and makes the regulator inefficient. The LM 350 is one such linear regulator. Depending on how large the difference between input and desired output voltage is, and how much current is drawn by the load, the LM 350 can get hot very quickly. A passive heat sink is very likely required. Linear regulators usually provide very steady output voltages and become much more efficient when the input voltage is close to the goal output voltage.
Switching Regulators
Switching regulators have more elaborate internal design and therefore usually more expensive. They are 'smarter' in how they convert the energy. They actively sample the input energy and buffer it in capacitors to output it at a different current level. Considerably less energy is wasted than in linear regulators, and therefore no cooling solution required. The Recom R-78xx series, for example, is a line of switching regulators with fixed output voltages. Switching regulators usually have a slight ripple on the output voltage.
There are regulators, like the Recom R-78xx series, which provide only output voltages in one of the standard values. There are also regulators, like the LM 350 or MP2307, where the output voltage can be adjusted by connecting resistors to specific pins.