Transmission Electron Microscopes (TEMs) are a bit different to SEMs. To look at something using a TEM the sample must be sliced into a very thin section and prepared specially. The electrons pass, or transmit, through the thin section of the sample, which is how the Transmission Electron Microscope gets its name.
Electrons are fired very fast towards the sample, just like in an SEM. Because the sample is so thin, when the electrons hit the sample, they pass through it! After passing through the sample, the electrons reach a screen where the image appears. The image can be seen on the screen itself, or on a computer screen.
A TEM can magnify a sample up to 50 million times (50,000,000x). This is far more than the SEM! However, it takes a long time to prepare a sample for TEM, which makes TEMs harder to use. The final image you see from a TEM looks 2D – it shows a thin section through your sample.