Paleozoic, Mesozoic, and Cenozoic refer to the three main eras of complex multicellular life on Earth -- in Greek, the words mean "ancient life," "middle life," and "new life." The Paleozoic stretches from 542 to 251 million years ago, the Mesozoic from 251 to 65.5 million years ago, and the Cenozoic from 65.5 million years ago to the present.
Speaking in extremely broad terms, the Paleozoic might be considered the age of invertebrates, land plants, amphibians, and synapsids (the ancestors of mammals), the Mesozoic was the Age of Reptiles (especially dinosaurs), and the Cenozoic is the Age of Mammals. Since the terms were coined, it has been discovered that complex multicellular life did actually exist about 60 million years before the start of the Paleozoic. This life is referred to as the Ediacaran fauna.
The Paleozoic began about 542 million years ago, when the land was devoid of life and all living things were aquatic. The oceans were filled with small, simple organisms such as trilobites, cnidarians (relatives of jellyfish), and quilted frond-like "mattresses" and "bags" which were the Ediacaran fauna. Quickly, the Ediacaran fauna went extinct and were replaced by a set of rapidly diversifying organisms which branched into early ancestors of nearly all modern-day animal phyla. This abrupt episode of adaptive radiation is now known as the Cambrian explosion.
Some of the most significant evolutionary milestones occurred throughout the rest of the Paleozoic. Animals grew from being an average of just a couple inches in length to forms much larger than humans, including the predatory fish Dunkleosteus, which grew up to 6 m (20 ft) in length and was the first vertebrate superpredator. Dunkleosteus lived in shallow seas of the Devonian period, about 370-360 million years ago. Life went from being entirely marine to colonizing the land, a process which began with early plants 475 million years ago or more, and wasn't completed until the earliest known trees began to form forests about 380 million years ago. The Paleozoic ended with the greatest mass extinction in history, the Permian-Triassic extinction, which wiped out about 90% of all animal species on the planet.
The Mesozoic was dominated by various types of reptiles, including the pterosaurs, which occupied the air and were the first flying vertebrates; the ichthyosaurs and plesiosaurs, marine reptiles which occupied the oceans; the sauropods, massive four-legged dinosaurs that were dominant herbivores; the theropods, meat-eating dinosaurs which walked on two legs and included Tyrannosaurus rex, and numerous other dinosaurs, which used armor and other adaptations to avoid being eaten. The flora and fauna of the Mesozoic have been well-publicized by various documentaries and films focusing on dinosaurs.
The Cenozoic, the most recent era, is characterized by mammals, which survived the mass extinction that killed the dinosaurs and went on to dominate the planet. Birds, which evolved from a group of theropod dinosaurs, evolved to take over the sky, while fish and whales occupied the seas. Mammals of all sizes evolved, though on balance they were smaller than the dinosaurs. Diverse lines of mammalian predators and herbivores evolved, culminating with the evolution of human beings, the most intelligent species the planet had ever seen.