LEDs are diodes that have a voltage drop designed to produce visible light. When using a LED in a cicuit it's important to use a current limiter such as a resistor. A typical LED is designed for aproximately 10mA. When using resistors measure the foreward voltage drop of the LED with a multimeter, subtract this from the supply voltage, then plug the difference into ohms law to calculate the required resistor value. Different color LEDs all have different voltage drops.
- Serial - These displays have a microcontroller on the display with a serial or I2C connection. Several builtin fonts are typically included. These displays are very easy to use, but the update rate for a display is very slow. Fullblown bitmap support is usually not available. Typical price for these displays is $50+.
- Parallel - Parallel displays connect to a microcontroller via a parallel interface( the bitwidth varies from 8bits to 32bits ). Displays can have a much faster update rate then serial since they allow arbitrary graphs to be display. The displays are typically much cheaper then serial display starting as low as $20.
Finding LCD displays is not easy since huge distributors such as Digikey have a limited selection.
Some places that do have LCDs for hobby use:
Sparkfun - Has a good selection of both parallel and serial displays. MatrixOrbitral - Specializes in serial display especially the type for direct connection to a PC. Jameco - A small selection. Mostly serial.