LEDs are available in many colors from visual Infrared, through the visible spectrum and into the near Ultraviolet. They are often listed as UV when they are really purple. It is hard to blame vendors for this as eBay lumps the two categories together. Most LEDs are made in China. Their color is very pure compared to incandescent bulbs. They can be turned on and off frequently without harming them, unlike fluorescent bulbs.

It is normal for vendors to use superlatives like "superbright", "Ultrabright", "Megabright" and so forth in their listings. You should completely ignore these words, they mean nothing here.

Millicandelas (mcd) are the unit of brightness most often cited in listings. Millicandelas are measured at a single point directly in front of of the LED. This is great if you only need a tiny point of light, but makes it difficult to meaningfully compare different LEDs. Lumens refer to the total amount of light put out by any light. Unfortunately many LED listings do not list this measurement

You could for example, have one LED that emitted 10 lumens of light, but spread them over a 100 degree angle. If you put this next to a LED that only emitted 1 lumen (one tenth as much light), but concentrates it into a 10 degree beam, both would have the same mcd rating.

A rough guide to translating between lumens and millicandela: http://led.linear1.org/lumen.wiz

LEDs are tiny little things that weigh almost nothing. Extravagant shipping fees are standard. It is common to pay $15 shipping for an order that could easily travel in a first class envelope.

Advantages:

Physically small. Long life. 100,000 hours, or ten years, is a figure often given for half life. Any light will dim with time. LEDs tend to dim very slowly. They will last even longer and be even more efficient when under driven (run with less than full power). They are essentially immune to cold, shock, and vibration. They have low energy consumption. They turn on and off more quickly than incandescent bulbs, which is useful for brake lights.

Disadvantages:

The beam tends to be more directional than that of an incandescent bulb. They are sensitive to small variations in current. A small increase in current can drastically shorten their life expectancy. 3, 5, and 10 mm LEDs won't melt themselves out from under snow like incandescent bulbs.

They have less "throw" than an incandescent bulb. They have a higher initial cost. There can be variation within and between batches. At this time they are not good for high output applications such as headlights and area lighting.

common sizes:

SMD (Surface Mount Device)

Little tiny LEDs too small for normal people to solder.

3 mm, 5 mm, 10 mm

bullet shaped, number refers to diameter of LED in millimeters. Has two leads at the flat end of the bullet. Very common.

Spider/Piranha

Flat square LEDs with four pins on the bottom. Usually have a wide beam. The extra legs give them extra mechanical stability.

1 watt, 3 watt, 5 watt

*"High Power" LEDs These units need to have a heat sink or they will overheat and burn out. These are the brightest LEDs currently in production.