Dinosaurs were reptiles which lived millions of years ago.
They evolved around 230 million years ago and lived in the Mesozoic period - known as the "age of reptiles".
Special scientists called palaeontologists (like Ross from Friends) study fossils and old bones to find out more about dinosaurs and how they lived.
There were more than 700 different types of dinosaur.
They are often classified as either meat-eaters (carnivores) like Tyrannosaurus Rex, or plant-eaters (herbivores) like Triceratops.
Dinosaurs all looked different. Some walked on two legs and others walked on four.
Some were very fast like Velociraptor but some were slow and lumbering, like Ankylosaurus.
Some had horns and spikes, or bumpy skin. But no one actually knows what colour or pattern dinosaurs were.
The word dinosaur means "terrible lizard" in Greek.
Lots of dinosaurs were named after Greek words which suited their personality or appearance. Some interesting ones are:
Velociraptor meaning "speedy robber"
Triceratops meaning "three-horned head"
Oviraptor meaning "egg-thief" (although it didn''t actually steal eggs)
Microdontosaurus meaning "tiny-toothed lizard"
Maiasaurus meaning "good mother lizard"
In Steven Spielberg''s Jurassic Park films, scientists bring dinosaurs back from the dead by using dinosaur DNA to recreate them.
But where could we get dinosaur DNA from?
Dinosaurs are loosely related to birds and scientists have claimed it might be possible to alter bird DNA to create a "Jurassic chicken" in 50-100 years'' time.
Dinosaurs died out around 65 million years ago.
There are several theories about why this happened.
The most accepted theory is that a giant asteroid crashed into earth around this time and caused catastrophic changes to the climate.
It was probably freezing cold and the dinosaurs could not adapt to the new weather conditions.