An atom has two main parts: A very tiny nucleus and the electrons that surround it. A very common picture that is often shown has the electrons going in circular paths around the nucleus.
This is partially true, since it shows that the electrons live around the nucleus, but it is also misleading since it gives you the idea that the electrons move in circular paths. In fact, the electrons do not move in nice orderly circles, or even in any paths at all. An electron should be thought of more as a fuzzy "cloud" that is able to fill the whole space around a nucleus, kind of like this picture: http://upload.wikimedia.org/wikipedia/commons/2/23/Helium_atom_QM.svg
This picture is supposed to give you the impression that the electron is all over the place at the same time. The darkness of the black color indicates that where the electron is most likely to be found at any given instant. As you see, it is most likely to be found close to the nucleus, and not so likely to be found far away. This is still not a very accurate picture, but it's better than the first one. In actual fact, for example, the nucleus should be drawn much smaller, so small that you would not see it. I'm sure if you look, you can find some other (better) pictures in your school library.
(published on 10/22/2007)