When compared to a light microscope, the transmission electron microscope (TEM) may seem similar in concept, but there are a few things that have been changed. The light source is replaced with an electron gun, and the glass lenses are replaced with electromagnetic lenses. The entire chamber through which the electrons pass is put under a vacuum, to avoid electron collisions with air molecules, and magnification variety is achieved through different currents in the electromagnetic lenses (as opposed to just moving the glass lenses in a light microscope).
The electron gun is usually composed of a heated tungsten filament, from which there is an electron cloud. This makes up the electron source, from which the electrons are pulled, accelerated, and then focused onto the sample. The sample has to be really thin (most are approx. 0.5 micrometers!) for the electrons to pass through and form an image. When the electron beam hits the sample, multiple things can happen. They (the electrons) can be absorbed, scattered, or even reflected; depending on the thickness of the sample and its composition. The image is formed on a fluorescent screen using the information gathered form the different actions of the electrons. This image can then be photographed for later use or documentation.
(see article link in "All You Wanted to Know About Electron Microscopy...But Didn't Dare to Ask!" post)