From Academic Kids
The Cenozoic Era (sometimes still Caenozoic in the United Kingdom) is the most recent of the four classic geological eras. It covers the 65.5 million years since the extinction event at the end of the Cretaceous that marked the demise of the last dinosaurs and the end of the Mesozoic Era. The Cenozoic era is ongoing.
Cenozoic is divided into two periods, Palaeogene and Neogene, and they are in turn divided into epochs. Palaeogene consists of Paleocene, Eocene, and Oligocene. Neogene consists of Pliocene, Pleistocene, and Holocene which is ongoing. Previously Cenozoic was divided into periods (or sub-eras) named as Tertiary (Paleocene to Pliocene) and Quaternary (Pleistocene and Holocene). However, they are no longer recognized.
The Cenozoic is the age of mammals. During the Cenozoic, mammals diverged from a few small, simple, generalized forms into a diverse collection of terrestrial, marine, and flying animals. The Cenozoic is just as much the age of savannas, or the age of co-dependent Angiosperms (flowering plants) and insects. Birds also evolved substantially in the Cenozoic.
Geologically, the Cenozoic is the era when continents moved into their current positions. Australia-New Guinea split from Gondwana to drift north and, eventually, abut South-east Asia; Antarctica moved into its current position over the South Pole; the Atlantic Ocean widened and, late in the era, South America became attached to North America.