the wavelength and the light source.
The light microscope uses light usually between 400-700nm wavelength. the light are photons.
the electron microscope uses electrons which are accelerated between anode and cathode have a wavelength of around 0.0040 nm. The much shorter wavelength of the electron beam makes that the electron microscope can resolve much smaller structures.
If you use google you can easily find a more detailed description. There are more differences, but the two mentioned are the most important ones.