This is the "Big" dog. And you can sort of see a dog shape in the constellation. This constellation also contains the well known star Sirius. It is famous for several reasons, though probably the most important reason is that Sirius is the brightest star in the night time sky - brighter to your eye than any other star out there (except for the Sun of course). Sirius is also called the "Dog Star", which may have to do with it being in the dog constellation. There is also the idea that Sirius is visible before dawn during the "dog days of summer", though that's not a very good reason - who gets up before dawn any ways? Canis Major is visible during the winter/spring months.