Birds are a type of dinosaur. Apart from details of the skeletons that I can't explain (because I don't fully understand them myself), the evolutionary adaptation that seems to mark the origin of dinosaurs is having feathers.

Non-bird dinosaurs used those feathers mainly for warmth, but birds have also evolved to use them for flight as well as warmth.

Warm-bloodedness is also something that all dinosaurs including birds share and their closest living relatives do not; however, there is evidence that crocodiles and alligators descended from warm-blooded ancestors as well, so warm-bloodedness was probably indicative of a larger group of reptiles than just dinosaurs.