Dinosaurs are not extinct. Although the mighty Tyrannosaurus won't make an appearance on your morning commute to work, you won't see a pack of troodontids leap over your back garden fence, and the vast herds of hadrosaurs are a thing of the past - dinosaurs still walk among us in the form of birds. All birds are descended from theropod dinosaurs.
This is not just a theory but a scientifically proven fact. Birds are not related to dinosaurs - they are dinosaurs. Avian dinosaurs. With the End Cretaceous, or KPg extinction, all of the non avian dinosaurs perished and became extinct, leaving behind a world of relatively small creatures. Among them were the feathered ancestors of the theropods, some of which fluttered about from branch to branch, some of which stalked the forests on long, earthbound legs. If you look outside your window right now, regardless of where you are in the world, you'll probably see a bird. Depending on your location, this could be anything from a chaffinch to a cardinal, a gull to a gnatchatcher, a buzzard to a bird-of-paradise.
Over ten thousand species exist today, and they compose, arguably, our most beautiful animals - brightly coloured plumages and beautiful songs have inspired naturalists, birdwatchers, poets, authors, musicians and painters over the course of human history, but it is only relatively recent science that places these amazing creatures in the same family tree as the now extinct, non avian theropod dinosaurs.