You need to current limit LEDs, so that's why the resistor. Depending on the circuit you may or may not need to current limit.
Pretty open ended question you're asking, how much do you already know about them and diodes in general?
The basic values you need to know are the forward voltage and current of the LED (or string of LEDs) you wish to use and the max supply voltage (e.g. the upper limit of your voltage regulator, usually assume about 14.5V). Typically LEDs work around 2-to-3V (you do not need to run them at max V, their output is exponential w.r.t. to bias voltage unlike incandescent bulbs) and in the 10-to-100mA range, so you'll be using values around 100-to-1KΩ with power ratings about 1/4 watt for single string LEDs.
"Silent enim leges inter arma." -- Cicero