LEDs

LED stands for light emitting diode.

They come in multiple forms and shapes. Here are some of the more popular sizes

Packages

3mm (T1) - very tiny also common

5mm (T1 3/4)- very common through hole led

10mm - uncommon, pretty fat led, awesome to play with

0805 - surface mount

1206 -

PLCC-2 -

PLCC-4 - often used in bicolor leds

PLCC-6 - often used in tricolor leds

millicandela mcd

10mcd nice moderate indicator

100mcd pretty bright indicator

1000mcd very bright, look for a diffusor if you intend to use it

5000mcd really bright, things start tasting blue

10,000mcd You shouldn't look at this directly or indirectly, just stop looking

mcd to lumen calc

http://led.linear1.org/lumen.wiz

Colors

625nm AlGaInP - 2.1V Brilliant Red

617nm - 2.1V Amber

590nm - 2.2V Yellow

527nm InGaN - 3.4V Brilliant Green

470nm InGaN - 3.4V Blue