The geologic time scale was created to represent Earth's long history. Scientists placed Earth's in order by relative age to create the geologic column. They developed the scale by studying rock layers and index fossils. It's kind of like a sand art bottle. The different colors of sand represent different layers of history. The sand on the bottom is relatively older than the sand that's on top and is considered young. Radioactive dating helps determine the absolute. For example, if you look at the Grand Canyon, you can visualize the multitudes of color change and layers. The oldest layers obviously lay at the bottom and the younger ones stack up way on the top. However, due to erosion, some layers wear off and in some rare phenomenons, the layers can completely flip around.
There are four types of time divisions: eon, era, period, and epoch. These are listed from 1st largest to 4th largest. We can say that geologic time began when Earth was formed, and will end when the sun demolishes the solar system.The divisions of the geologic time scale depend on the history of life on the Earth at that moment. Every time period had a different change in its atmosphere and environment which evolved life. It's crazy to see how much the Earth has evolved.