Earth's existence is separated by geologists into even longer times called eons. A number of eras make up an eon.
We are now in the Cenozoic Era of the Phanerozoic Eon. Cenozoic is from a Greek word meaning new life, that is, the time of the newest life on Earth.
The Phanerozoic Eon started about 541 million years ago. It is made up of the Paleozoic (from the Greek for "oldest life") Era, the Mesozoic ("middle life") Era and the Cenozoic Era.
The Paleozoic Era started about 541 million years ago, then the Mesozoic Era started about 251.1 million years ago. This was followed by our era, the Cenozoic Era, which started about 64.3 million years ago.
Era is also used for parts of human history, for example the Napoleonic era.