The Mesozoic is one of four (sometimes more) Geologic Eras. The division of time into Eras dates to the 18th Century.
The Mesozoic includes three Geologic Periods. From oldest to youngest -- Triassic, Jurassic and Cretaceous. It extended from roughly 245 million years before present to roughly 64 million years. It follows the Cambrian Era and is followed by the Tertiary Era.
The Mesozoic covers the time when life was dominated by large sophistocated reptiles. The lower (oldest) boundary was set at a major extinction event at the end of the Permian Period. The upper (youngest) boundary is set at a second major extinction event 64 million years ago that marked the demise of the dinosaurs, of most marine reptiles, of shelled cephalopods called ammonites, and of many other forms.
Geologically, the Mesozoic starts with almost all the Earth's land collected into a supercontinent called Pangea. During the Era, the Pangea split first East to West into a Norther Continent Larussia and a Southern Continent Gondowanaland. Later those continents split up with Larussia splitting into North America and Eurasia. The Southern continent broke up progressively into four continents -- South America, Africa, Australia and Antarctica.
The Mesozoic is known as the Age of Dinosaurs. It also saw the development of early birds and mammals and of flowering plants. At the end of the Mesozoic, all the major body plans of modern life were in place although in some cases -- notably the mammals -- the forms that existed at the end of the Cretaceous were quite primitive.