The only real difference between a diode and an LED is the material it's made of sine an LED is actually a light emitting diode so the led really is a diode as well.
A diode is made of Indirect Semiconductor (Si, Ge) materials while an led is made from direct semiconductor material (GaAs. InP).
They're essentially the same thing but one is made of different material than the other depending on what the diode is being made into.