Geologic time is the history of our Earth divided into 4 different Eras, whereas each Era is marked by special events, such as the emergence or extinction of a particular species.
The 4 major eras of geologic time are: The Precambrian, Paleozoic, Mesozoic, and Cenozoic Eras.
But how can scientists come up with such a timeline? Scientists find fossils in different layers of rock (whereas the bottom layers are the oldest). They study these fossils and also observe how they change from one layer of rock to another. Therefore they can then use relative dating to divide our planet's rich past into different chunks, according to these studies. Scientists also make use of absolute dating, i.e. given a fossil with no particular information about where it was found, they can date it quite accurately.
The geologic time scale is divided into eons, eras, periods, and epochs.
Have you ever wondered how it can be that fossils of a particular land species, such as dinosaurs, were found on every continent? How could these species have possibly crossed continents?
The answer to this is: Pangea! During the late Paleozoic and early Mesozoic eras (around 300 million years ago), all continents were joined into one super continent, called Pangea. How do we know? One of the evidence that we have are fossils! Fossils indicated the presence of identical species on continents that are now great distances apart!