The Mesozoic is the time in the Earth's history from 245 million years ago (Ma) to 65 Ma.  It is divided into three periods:

                                  Cretaceous (146-65 Ma)
                                  Jurassic (208-146 Ma)
                                  Triassic (245-208 Ma)

    Mesozoic means "middle animals", as it is the time after the primitive animals of the Paleozoic and before the advanced animals of the Cenozoic (today).  The dinosaurs were the dominant animals of the day.  They evolved in the Triassic, but did not become highly diverse until the Jurassic.  All dinosaurs became extinct at the end of the Cretaceous.  Plants also became important in the Mesozoic.  During this time, modern gymnosperms and early angiosperms developed.

     It was in the Mesozoic that the dinosaurs left there tracks.  In fact, the earliest evidence of dinosaurs is from tracks, not a body fossil.