The urinal represented a major shift in modern art, as Duchamp was really raising two fingers to the art establishment, which of course, eventually embraced it. Metaphorically.
Not only is it art, but really a key icon in 20th century art, (and probably the most important art-work of the modern era), which shattered many preconceptions, and led the way forward to conceptual art, installations, performance art, readymades, and the like.
Quote from Stephen Hicks: 'The artist is a not great creator—Duchamp went shopping at a plumbing store. The artwork is not a special object—it was mass-produced in a factory. The experience of art is not exciting and ennobling—at best it is puzzling and mostly leaves one with a sense of distaste. But over and above that, Duchamp did not select just any ready-made object to display. In selecting the urinal, his message was clear: Art is something you piss on.'