It's easy enough to get Art Nouveau and Art Deco confused, probably owing to the fact that they both start with 'art'. But Art Nouveau and Art Deco are actually two very distinct design movements, with very distinct looks, that appeared around the turn of the 20th century. After perusing this brief disambiguation, you may not be an expert on design history — but you can casually drop these two descriptors into conversations, and your friends are sure to be impressed.
Art Nouveau was a design movement that began in Europe around 1890. It was in part a response to styles popular earlier in the 19th century, like Neoclassicism, that heavily referenced historical styles. Art Nouveau designers wanted to create an entirely new design vocabulary appropriate to the modern world. The movement was heavily influenced by the paintings (like the one up top) of Czech artist Alphonse Mucha, so much that it was sometimes called 'Mucha style'.
Art Nouveau is characterized by whiplash curves and motifs inspired by nature, but, as our design historian Anna points out, more by the vital force of nature than its delicate prettiness.
The designs often feature sensuous portrayals of women that would have been considered scandalous at the time. All these elements taken together mean that Art Nouveau designs often have a dreamlike, otherworldly feel.
Around 1910 Art Nouveau began to be replaced by Art Deco, which in many ways was Art Nouveau's opposite, characterized by geometric forms, expensive materials (lacquer, ivory, gold), and exotic motifs inspired by Chinese, African, and even Mesoamerican design.
In France, the movement emphasized exclusivity and luxury: in America, it became more democratic, a celebration of the newfound potential of new forms of transportation and the machine age. The human body was depicted in a very stylized, idealistic way, which might help explain why pretty much everything Art Deco looks, to me, like it belongs on the cover of an Ayn Rand novel.
I think the most interesting thing about them is they both feel completely fresh and original, but also form a bridge between the styles of the 19th century, like Romanticism and Neoclassicism, which feel very old-fashioned to us, and Modernism, which feels very, well, modern. Googling (or Pinteresting) either of these terms is a wonderful way to lose yourself in a pleasantly alien but fully realized world.