Theropod means 'beast feet' and refers to the main hunting dinosaurs. All had three toes.
The earliest theropods date back to the start of dinosaur times. Strangely, perhaps, these were the dinosaurs that eventually became the flying birds we now have all around us, not the flying reptiles of the say. So, T. rex, the most famous of them all, might have evolved into one of the most easily-frightened animals of all - the chicken!
Theropods look immensely tough and bulky, so you might imagine they weigh as much as a tank, but although thy are heavy, they are light for their size because they have air-filled bones, something else they have handed down to birds.
Of course, many people love to know about giant monsters like T. rex. Since the discovery of T. rex, even bigger hunters have been discovered, including Spinosaurus ( for which good remains have been found, and which was 6m longer and 1 tonne heavier than T. rex) and Giganotosaurus (for which only poor remains have been found).
Funnily enough, the earliest hunters, such as Eoraptor, were very small. As they evolved they got bigger and bigger and bigger. Most theropods were meat-eaters.