^No problem it is a good question, but asking me would result in a situation of the blind leading the blind. 
I came across the discussion below from google search. It came out on top of the search and I believe this means itās read by a lot of people (?). I bold faced the parts that seem relevant to your question.
BTW may I ask you why youāre asking this question (in case someone more knowledgeable than me could help)? Are you choosing lights based on the driver technology used? Hope this helps.
Answer by poster ādingwatā:
Iāll try to answer this is reasonable concise post but this is a huge subject (Iāll probably have to add to edit a few times so bear with me). If any of the terms/abbreviations/initialisms arenāt clear, let me know. Iād love to provide more schematics, but many ādriver peopleā do not release schematics for reasons I donāt fully understand.
First off, what are we trying to do with a ādriverā? For LEDs, a flashlight driver is a constant-current (CC) source: LEDs are happiest when driven CC, as opposed to CV (constant-voltage) or something else. Hereās a comment of mine attempting to give a short explanation of why. Most white LEDs have a forward voltage (Vf) around 3.0V, but some LEDs have multiple dies connected in series (e.g. Cree XHP50, or Nichia 144A) which increases the voltage. Also note that with LED Vf and battery voltages below, Iām referring to nominal, the min and max may be significantly different: a Li-ion battery has a nominal voltage of 3.7V, but is typically operated over a range from 4.2V (full charge) to 3.0V (mostly discharged).
Switching converters
Both buck and boost are switching converter architectures (there are others, but they are not as commonly encountered in flashlights). A well designed switching converter is very efficient, but they are complex and may be expensive. Most buck and boost converters in flashlights have a MOSFET (operating as a switch), an inductor, a diode, and a capacitor. The diode can sometimes be replaced with another FET to improve efficiency, and the capacitor can be left out in some situations (particularly in smaller LED drivers), but the FET and inductor will always be present. Switching converters are true regulators, and (if well designed) will drive an LED at a fixed current regardless of the battery voltage until the battery cannot provide enough current, resulting in very flat runtimes (no brightness variance over time).
Boost
A boost driver is a CC switching converter , with a lower voltage on the input than the output. For example:
1.5V to ~3.0V (for a alkaline/NiMH to single emitter driver, like)
3.7V to ~6.0V (for a Li-ion to two series emitter driver)
3.7V to ~12.0V (for a Li-ion to four series emitter driver)
A boost converter (both CC and CV) is, at itās core, a switch that, when turned on, pulls current through an inductor, and when the switch is turned off, the current flowing through the inductor doesnāt want to stop (thatās the defining characteristic of the inductor) so the voltage increases, and a diode then prevents the current from turning around again. There is generally a capacitor at the output as well, but thatās not strictly necessary depending on the load. This Wikipedia image illustrates it very well.
Buck
A buck driver is a CC switching converter (again, has a FET and inductor), but the voltage in is higher than the voltage out. For example:
3.7V to ~3.0V (for a Li-ion to single emitter driver, although this has some complications)
7.4V to ~3.0V (for two Li-ion to a single emitter)
A buck converter (both CC and CV) is essentially a switch which is turned on or off by a control loop that looks at desired voltage or current versus the actual voltage or current, with a diode to allow current to flow when the switch is off, followed by an LC (inductor-capacitor) filter to smooth the alternating āall currentā/āno currentā into āsome currentā. Again, the Wikipedia image is an excellent explanation.
Linear
In the context of a flashlight driver, linear refers to a constant-current linear regulator. The most commonly used part is the AMC7135, a 350mA current regulator. These devices are very much like a linear voltage regulator (e.g. LM7805), except they regulate current. They generally use a bandgap reference controlling a pass element. They are called linear because the pass element is a transistor (usually a BJT or a MOSFET) operating in the linear region (meaning itās not all the way on or off). They are very simple to design in, but cannot handle very much current per device (so larger drivers have many e.g. Convoy S2+ driver has 8x7135 paralleled together). They are technically much less efficient than switching converters because they dissipate the extra voltage across the pass element, but when used with a low voltage drop (going from 3.7V to 3.0V, for example) and at low currents, they can be quite efficient and may be more efficient than some switching designs due to the high quiescent current of the switcher.
Hereās a schematic of the āNanjg 105cā driver from many Convoys, from this thread on EEVBlog.
FET
Calling a FET a driver is a stretch. A āFET driverā is really just a big MOSFET in series with the emitter, operating as a switch (i.e. the MOSFET is either in saturation/active or cutoff). When itās on, the full voltage of the battery minus the voltage drop of the wires and components is put across the emitter. This is typically used when the input voltage is close to the LED forward voltage (3.7V to 3.0V), and the current in a FET driver is generally high enough that the internal resistance of the battery plus the resistance of the wires, springs, PCB and the Rds(on) of the FET greatly affects the voltage at the emitter. Consider: at 20A, 50mĪ© results in a drop of 1V, so the Rds(on) and the springs are very important in how much current the emitter sees. Unlike the above regulators (switching and linear), a FET is not regulated, so as the battery voltage decreases, the LED brightness will also decrease.
The reason I say a āFET driverā isnāt really a driver is because the only mechanism ācontrollingā current through the emitter is the resistances mentioned above, a FET driver is literally just a switch that goes āhey there emitter, Iām gonna give you as much current as the system can provide whether you like it or notā.
Linear and FET are commonly combined in one driver, the linear element(s) for high efficiency and simplicity on low output, and the FET for driving the LED as hard as it can on high modes. These are frequently called āFET+1ā or āFET+Nā meaning a FET plus some number of linear regulators (generally AMC7135). These drivers have a restriction that voltage in must be greater than voltage out by a small amount, determined by the dropout voltage of the linear regulator (for instance, the 7135 has a dropout voltage of 120mV, so the driverās input voltage must be at least 120mV greater than the Vf of the LED.
Others
There are other types of DC-DC converters, but they are less commonly seen and/or rarely used for flashlights or constant current applications. These include buck-boost (inverting and four-switch), SEPIC, Äuk, flyback, and AC-DC SMPS designs.
You also mentioned a series resistor, which is, in a way, an LED driver. A series resistor can be used to drive an LED when there is a constant Vin (or nearly constant) greater than the LED Vf. The resistor is chosen such that the resulting voltage drop through the resistor equals the difference in voltage between Vf and Vin at the desired brightness (desired current). Depending on how big the voltage difference is, and the amount of current through the emitter, this can be a very large amount of power, and therefore this method isnāt really usable for anything but very low output emitters and a suitable battery. Also note that as Vin decreases, so does the LED brightness.
ā¦my fingers are tired, Iām going to go make potatoes.