In terms of their appearance: art nouveau applies soft, filigree patterning; often impersonating vines and vegetation in it's style. Nouveau generally incorporates much more detail and has an overall feminine appearance & aesthetic in its style.
Art deco is generally in direct contrast with nouveau: deco has harder more ridged contours, generally applying a variety of sharp angles in the way of triangles, columns, blocks and step designs; however, deco will incorporate some hardened & galvanized curves and undulation (like that of a theater curtain) in much of it's style. It is a harder style than nouveau and has been often associated with masculine aesthetics - this is evident in Nazi Germany & Soviet Russian graphic design of the 1930's - 40's.
art nouveau is wrongly thought to have originated from France; nouveau has been found throughout the world such as that Tsarist Russia. Defining Art deco is more allusive as there is virtually no single origin of nationality. Deco borrows from Mediterranean, Moroccan, Aztec, Egyptian & African styles just to name a few. It's no wonder deco found popularity in the early 20th century when the pursuit of exploration and discovery was all the rage. While we describe deco as worldly, it has however, been said that "if one tries to define art deco, then one has missed the point of art deco". There's something to said in this about its dynamic & amorphic orientations. I can recommend looking up art deco in google image search alongside a country of origin and comparing those styles alongside art nouveau.
This is brief and generalized description, but I hope it helps :).