LiveScience has an interesting story that explains how and why birds are dinosaurs.  

What do sparrows, geese and owls have in common with a velociraptor or the mighty Tyrannosaurus rex? All can trace their origins to a bipedal, mostly meat-eating group of dinosaurs called theropods (“beast-footed”) that first appeared around 231 million years ago, during the late Triassic Period. 

The earliest birds shared much in common with their theropod relatives, including feathers and egg-laying. However, certain traits – such as sustained, powered flight – distinguished ancient birds from other theropods, and eventually came to define modern-bird lineage (even though not all modern birds fly).

Today, all non-avian dinosaurs are long extinct. But are birds still considered to be true dinosaurs?