Hi Guest
That is a very complex question, probably much bigger than you think
The short answer is that during the Mesozoic dinosaurs split up into a multitude of groups that all changed in different ways.
In the dinosaur family tree below you can see how the first dinosaurs (bottom) split into ornithischians (left) and saurischians (right). Saurischians further split into sauropods and theropods.
All of these groups radiated further into various different species, but we
can track some general changes - some changes even repeat themselves in different dinosaur lineages.
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Dominance: As the first change we should note how dinosaurs went from being a somewhat insignificant group among many others in the early Triassic, to become the dominant group of land animals - a title they kept for around 200 million years! There where many species of insects, reptiles, crocodiles and mammals living alongside the dinosaurs, but apart from a few supercrocs, dinosaurs ruled the earth.
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Size: The first dinosaurs were only around 1 meter(3 feet) long (
Eoraptor, Pisanosaurus, Eocursor) but with time giants evolved in all of the major groups.
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Diet: The first dinosaurs show signs of omnivory (eating whatever they could find; insects, meat, carcasses, nuts and seeds) but as time went by the ornithischians specialized in plant eating and so did the sauropods. The theropods became specialized meat eaters, with the exception of a few groups that went back to omnivory (early therizinosaurs and troodontids for instance). Some beaked theropods even went completely vegetarian (
Limusaurus, ornithomimosaurs, the advanced therizinosaurs, and many species of birds)
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Beak: Dinosaurs started out with teeth going all the way from the front and out into the cheeks. But many groups gradually replaced some or all of their teeth with a beak. Both the ornithischians and a number of theropod groups (
Limusaurus, the ornithomimosaurs, the therizinosaurs, the oviraptors, birds, and (if I'm not mistaken) Alvarezsaurs)
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Lungs: Different lines of evidence now tell us that the first dinosaurs had a super efficient, tubular lung system with unidirectional airflow. Today best known from birds but also seen in alligators
http://news.nationalgeographic.com/news/2010/01/100114-
alligators-dinosaurs-birds-lungs-breathing/
Skeletons of dinosaurs show that both sauropods and theropods had air sacks connected with their lungs along the neck and backbone. During the Mesozoic both groups evolved to have these air sacks growing into the bones of the neck and spine making their bodies light and their breathing better. The final stage of this development is seen in birds, which have the best lungs of all animals and air sacks growing into some 80% of their entire skeleton!
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Feathers: Theropods evolved the first feathers. In the early stages it looked like hollow hairs, then downy feathers evolved, and finally true feathers were invented by the group called maniraptors (top, right in the family tree above, represented by
Dinonychus and a pigeon).
The Pterosaurs, flying reptiles closely related to dinosaurs, also had a fur of hollow hairs and two different ornithischia fossils (
Tianyulong and a
Psittacosaurus) are preserved with what could be fur or bristles down their back and tail. Researchers still discuss if this means that the first dinosaurs all had hollow hairs and that some of them later changed it into feathers while others gave it up (fossil skin of hadrosaurs, ceratopsians, and tyrannosaurs show only scales, no fur/feathers).
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Birds: The thing I find the most amazing about dinosaurs is how some of them changed into what we call birds today. The first bird-like dinosaurs show up in the late Jurassic (
Anchiornis, Pedopenna, Epidexipteryx, Epidendrosaurus, and
Archaeopteryx) and through the Cretaceous birds as we know them evolved; with beaks, perching feet, short tails, and the ability to fly. Their theropod ancestors already had many of the features we normally assign to birds: Feathers, air sacks, small light wheight skeletons, whish bones, warm bloodednes and so on. Some maniraptors could even glide from tree to tree (
Microraptor in particular). But birds took all these things and used it for something amazing: powered flight.
That was a bit of a mouthful, but I hope it's useful to you
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