The earth is approximately 4.5 billion years old. Life appeared in the oceans about 1 billion years later, but it wasn't until about 570 million years ago that the major groups of multi-celled organisms appeared. This was the beginning of the Phanerozoic Eon, which continues to the present day.
The Phanerozoic is divided into three major eras of life: the Paleozoic ("ancient life"), the Mesozoic ("middle life") and the Cenozoic ("new life"). Dinosaurs lived during the Mesozoic Era, which lasted about 180 million years.
The Mesozoic Era (known sometimes as the Age of Reptiles or Age of Dinosaurs) is divided into three time periods: the Triassic Period, the Jurassic Period, and the Cretaceous Period. Dinosaurs appeared toward the end of the Triassic, 225 million years ago. Initially, they were small in size and ran on their hind feet. During the Jurassic Period dinosaurs greatly diversified and many of them became much larger in size. This was the age of such famous dinosaurs as Brontosaurus, Stegosaurus, and Allosaurus. These dinosaurs disappeared and were replaced in the Cretaceous by other well-known forms such as Tyrannosaurus, Triceratops, and Edmontosaurus.
The last dinosaur (not counting birds) disappeared 66 million years ago at the close of the Cretaceous Period. At the geologic boundary between the close of the Age of Dinosaurs and the beginning of the Age of Mammals (Tertiary Period in the Cenozoic Era) there is a high concentration of the rare earth element Iridium. This has been associated with a large comet or meteorite impact that occurred at that time and may (at least partly) explain the demise of the dinosaurs. The boundary between the Age of Dinosaurs and Age of Mammals is sometimes referred to as the "K/T Boundary" (pronounce it K T Boundary). The letter "K" is used by geologists to designate the Cretaceous Period and "T" represents the Tertiary.