It’s useful to think about two main types of LEDs—the familiar indicator LEDs that come in 5mm and 3mm epoxy packages, and “illumination-grade” LEDs, which are high-output devices, designed for lighting.
A typical indicator LED has a forward voltage rating between 2 and 4 Volts of DC. You may see maximum ratings above that. A typical drive current for indicator LEDs, even high-brightness ones, is 20 milliamperes (mA). From this you can see an indicator LED dissipates a modest amount of power—a few tens of milliwatts compared to the few tens of Watts a familiar incandescent bulb uses. In other words, the power used by an indicator LED is one thousandth of that used by a familiar light bulb.
Arrays are constructed to take advantage of this low power consumption. A series string of ten blue LEDs will take a 33 VDC forward voltage to light, but still only draw 20 mA of current from the source. So the supporting wiring can be less expensive for an LED array compared to a light bulb (which may draw an Ampere of current—fifty times as much as the LED). For parallel arrays, the current combine. So you can drive 10 blue LEDs in parallel from a 3.3V source, but the current drawn will be 200 mA. This flexibility in array construction is part of what makes LEDs very popular in mobile, battery-powered devices. The designer can arrange LEDs to take best advantage of the power source that’s available.
Illumination-grade LEDs have comparable forward voltages to indicator LEDs. This is a reflection of the fact that the junction material is the main determinant of the forward voltage. But the junctions in illumination-grade LEDs are typically larger, and can draw more current, and dissipate more power (while producing more light). A Luxeon Star LED has a drive current of 350 mA, and dissipates about 1 Watt of power.
Another important LED spec is maximum reverse voltage. A diode conducts current when a forward voltage is applied, but will not conduct if a reverse voltage is applied, up to a point. Reverse voltages in excess of the maximum can cause the diode to fail.