The graphic below is a representation of the geologic time scale used by geologists (we bent it and chopped it off a little bit). Time periods in the earth's history are given names, so that they can be easily referred to. The boundaries between each unit are commonly based on changes in the fossil record.
Move your cursor along the timeline from the the upper left to the lower right. The name and common character code for each time period are displayed. Just click on any of the periods to see some of the flora and fauna that existed during that time.