I'm sorry I missed out on this thread until now. I guess I need to come here more often.
Anyway,
There are two ways to define dinosaurs. One of them is a descendent of the common ancestor of all dinosaurs, which is a definition that birds fit since they're descended from dinosaurs. But birds fit the other definition of dinosaurs also: all of the physical traits that unite dinosaurs and set them apart from other groups of animals are also present in birds.
I explained what some of these traits are at
http://www.christianforums.com/showthread.php?p=2556389#post2556389 .
And just to clear up some of the other confusion about what a dinosaur is: this set of traits is not present in crocodiles, pterosaurs, plesiosaurs, mosasaurs, or any type of lizard. There are also no transitional fossils between dinosaurs and these animals, apart from fossils of the group of reptiles from which dinosaurs, pterosaurs, and crocodiles are descended. So on the basis of both anatomy and cladistics, no animals alive today except birds deserve to be called dinosaurs.
