LED' are neither 3.3V or 5V, with the rare exception of LED's which have an internal series resistor calculated for 3.3V or 5V. A "5V LED" can still be connected directly to a GPIO, but its brightness will be diminished.
LED's are actually Diodes, so they need a certain voltage before they start to conduct, (the "forward voltage") this voltage mostly depends on the LED's light wavelength, with longer wavelengths (shifting to RED) associated with lower forward voltages (around 2V) and LED's with shorter wavelengths (shifting towards BLUE) having somewhat higher forward voltages (around 3V).
As DIODES they need a series resistor to actually limit the current flowing through them, typically from 1mA (high efficient) to 10mA (some older LEDs). To calculate the resistor voltage you take the feeding voltage, say 5V, and subtract the LED's forward voltage, so you get the voltage over the resistor. Then its just Ohms law, R=U/I (or R=V/I depending on which notation you use).
example a high efficiency RED LED on 5V. So:
V = 5V
V Forward = 2V
I = 1/1000 A = 1mA = 0.001A
thus R = (V - V Forward) / 0.001 = 3 /0.001 = 3000 Ohm, or 3K