Geological Time Scale

Question 1:

I think the three major geologic events in the history of the earth were Jurassic, Sirulian, and Precambrian.

Question 2:

There are two eons, which are phanerozoic and precambrian. For  era there is four cenozoic, mesozoic, paleozoic, and proterozoic;  as well as eleven periods and seven epochs.

Question 3:
The difference between eon and era:
Eon: An indefinite and very long period of time, often a period exaggerated for humorous or rhetorical effect.
Era:A long and distinct period of history with a particular feature or characteristic

Question 4: 
Scientist created eons and eras so we'd be able to understand and clearly see the events that ocured in the past.