This site will also help clarify cladistics. Terrestrial Impact Craters has pictures and explanations of when, where, and what happened when meteors hit the Earth. The Case of the Mass Extinction is an interactive site which asks you to evaluate data. Mass Extinctions gives information on all known mass extinctions. How fossils are formed - soft parts of the body usually decay, leaving nothing behind.
Older fossils are on the bottom, younger fossils are closer to the top and most recent strata.
This is only useful at a single location. Absolute age can be determined by radiometric dating. This allows fossils from different sites to be compared. There are several radio-isotopes that are useful. Carbon 14 is used for fossils under 50,000 years of age and Potassium 40 can be used for fossils hundreds of millions of years old.