LEDs
Light-Emitting Diodes are not like incandescent lamps, and they do not behave like resistors that heat up. Accordingly, they cannot be used in the same way as flashlight bulbs or mini-bulbs. An incandescent flashlight simply connects each side of the battery to the bulb which then lights up automatically. If we connect an LED in the same way, it is likely to burn our almost immediately.
As an incandescent lamp filament heats up, it increases in resistance and reduces the current draw. Having a filament that automatically varies in resistance tends to reduce sensitivity to battery voltage. But LEDs have no filament and typically require some sort of external current-limiting device, whether a resistor or a more complex driver.
LEDs have been used in equipment for decades, and generally have had a resistor or on-chip current source for each and every LED lamp or numeric segment (in displays, the current typically is multiplexed between digits, one digit at a time). An easier alternative for portable situations is to use batteries which simply cannot source more current than the LEDs can handle. The power source limitations of little AAA or watch batteries can be used advantageously in LED flashlights. And, if we can find a bunch of LEDs which have nearly the same forward voltage, we can put them in parallel, drive them with a somewhat larger battery and get a lot more light, still without using discrete resistors. In essence, for some applications, the current-limiting we need is built into the battery. 
Diode Technical Limits
Light-Emitting Diodes are a form of semiconductor technology. A diode is a device with two terminals that generally allows electrical current to flow in only one direction, after a certain voltage has been reached. Although the diode directional properties are the reason most diodes exist, they are almost a side-effect for LEDs, which produce light only when current flows.
Forward Voltage
A certain amount of voltage is required to make a diode conduct. However, once the diode forward voltage is exceeded, conduction immediately occurs to the full extent the external circuit will allow. The lack of an inherent LED property to limit current to reasonable levels is the main reason for the differences between incandescent and LED circuits.
The forward voltage drop times the amount of current flowing is the power available to an LED to make light. Typical LED forward voltage may be about 2 volts (for red, orange and yellow lamps) or 3 volts (for green, blue and white lamps), but will vary widely with manufacturing, current and temperature.
In a series circuit, the voltage delivered onward is lowered by the forward voltage for each diode involved. To control the current we only have to deal with the remaining voltage. Then we can set the current with a resistor.
Forward Current
Too much current in the forward direction can melt the semiconductor junction, thus permanently destroying the device and forming a short circuit instead.
LED forward current when operating in these strings is typically around 20 milliamperes (mA) or 0.02 amps. So the red, orange and yellow lamps each use about 40 milliwatts (mW), while the green, blue and white lamps use about 60mW. In contrast, an incandescent bulb would use about 400mW, and probably deliver less light.
Maximum Reverse Voltage
All diodes have a limit to the voltage they can resist in the opposite direction, and when that voltage is exceeded, reverse current will flow. This is called junction breakdown, and can be nondestructive, as long as small currents are involved.
Typical LED reverse voltage ratings are only about 5 volts or so, although actual devices may withstand more. The 5V value does not quite guarantee that a 30-lamp circuit will withstand the 168V peak of a 120VAC line, but a 6V value would.
Maximum Reverse Current
Diodes are vulnerable to small amounts of current in the reverse direction during junction breakdown. Allowable reverse current ratings can be low as 50 microamps (uA). Levels substantially higher than the specified value are another way to destroy a diode junction.
