Original: It is the youngest period of the Mesozoic Era, and at 80 million years long, the longest period of the Phanerozoic Eon.
Simplified: The Cretaceous period is the longest period in the Phanerozoic eon, lasting from 145 to 65 million years ago, a total of 80 million years. It is the last period in the Mesozoic era.