What is an electron microscope?

Answer 1

An electron uses electrons to "see" an object, rather than light. The problem with light, is that it has a comparatively large wavelength. This is fine when you're looking at something relatively big (like a microorganism or a whole cell), but once you get down to very small structures, the wavelength of the light can be larger than the object that you're trying to examine.

As an analogy: imagine you're trying to examine something with your eyes closed (lets assume that for whatever reason, you can't touch it with your hands), so you get something to poke it with. Now imagine all that you have on hand is a large stick. You could poke the object with the stick and get a general idea of it's size and shape, but very small details will be lost. Now imagine that you have a very fine, thin stick. With it, you'll be able to determine much more precise detail and small objects. The large stick (regular light) is good for observing large objects without very fine detail. The small stick (electrons) is best for observing very tiny objects or ones with fine detail.

You get much better resolution with electron microscopes, but they do have their down sides. First, they are very expensive and difficult to maintain. Whereas a light microscope uses lenses to focus light, an electron microscope uses magnets to focus an electron beam. These magnets are very expensive, draw a lot of electricity, and are prone to overheating.
Also, electrons tend to zip right through small or thin objects. It is easy to stain a cell for observation under a light microscope, but in order to view something with an electron microscope, you need to stain it or coat it with a heavy metal that blocks electrons, such as silver or gold. These stains are difficult to use, and can result in what are called artifacts, or false details (if a blob of stain collects inside a cell, it can be difficult to determine if it is an artifact or if it is something interesting inside the cell).

Answer 2

I'll give you the same source again... excellent in reference to both your questions:

http://www.southwestschools.org/jsfaculty/Microscopes/types.html