The concept of Gesamtkunstwerk is the most important thing you need to know to understand and know about in Art Nouveau style architecture, fine and decorative arts. The word Gesamtkunstwerk is a German term that means total art work. The goal of Art Nouveau artists, designers and architects was to elevate every material object to the status of fine art and to design buildings, furniture, clothing and other material goods so that they all were in harmony. This meant that the details and aesthetics of every object surrounding you, and even your own body in some cases, were supposed to match.
The term Art Nouveau was invented by a German art dealer in Paris in 1895 when he opened a gallery called L'Art Nouveau, which translates literally to "the new art." This dealer's idea was to display a totally new style of art work appropriate for the modern age. This style incorporated curving, tendril like lines and applied not only to fine arts, but also decorative arts like furniture and dishes. Art Nouveau reached an international audience when it was introduced at the Paris World's Fair in 1900. You can find Art Nouveau buildings and objects from all over the world now, but most especially from Western Europe and the United States.
You can usually recognize Art Nouveau objects and buildings by their elaborate ornamentation, as the point of Art Nouveau was to create an international style based on decoration. Art Nouveau stuff is always covered with whiplash curves, intricate arabesques and abstracted botanical representations. Art Nouveau was in many ways a reaction to the industrial age. Art Nouveau artists and designers idealized nature and sought to represent it in their work. Most Art Nouveau stuff features a lot of flowers and plants or women in flowing gowns with long, unkempt hair. Artisans and artists were especially interested in recent scientific advancements by people like Charles Darwin that suggested that human beings were part of nature rather than constantly struggling against nature. Part of this idealization was certainly a negative reaction to industrialization and mass produced goods. Some proponents of Art Nouveau were especially interested in returning to handicraft. For example, very nice wooden Art Nouveau furniture is hand carved and painted.
Ironically, a lot of this Art Nouveau furniture, architecture and design that was celebrating nature were only made possible by technological advancements. Art Nouveau designers loved elaborate, intricate wrought iron fixtures like the Metro gates in Paris. These things were made with cast iron, which was a recent invention and allowed artists to make all sorts of new, intricately detailed shapes. You can see examples of Art Nouveau cast iron work in the bases of Tiffany lamps. Floral cast iron was considered to be especially modern because it required the latest in technology to produce, and it also espoused the latest in scientific and philosophical theories about man's relationship to nature.
This idea that man was a part of nature led lots of Art Nouveau designers to create imagery of metamorphoses. Human forms would be fused with plant or animal forms. Thus you can find Art Nouveau dishes or murals that feature women turning into flowers or running with herds of deer. In an extreme example, American dancer Loie Fuller created a routine in which she danced so quickly, garbed in flowing veils, that she seemed to turn her body into a flower. Art Nouveau designers sought to recreate stylized scenes of nature in interiors, so sometimes thinks like wooden wardrobes are carved to resemble trees. In fact, the most talked about item at the 1900 World's Fair was a lady's brooch that looked like a dragon fly.
Despite this obsession with nature, Art Nouveau was primarily an urban style. Art Nouveau buildings and furniture, in general, were enjoyed by the upper class in cities like Chicago, Paris and Vienna. Each of these cities developed their own version of Art Nouveau style, so if you wanted to collect some Art Nouveau furniture today you might want to choose a city with the style that most appeals to you. Paris and Chicago, for example, produced mostly the curvilinear design that you might be most familiar with. Designers in Brussels and Glasgow, on the other hand, were more likely to produce geometric designs that presaged art deco.
Unfortunately, the horrors of World War I caused most Americans and Europeans to lose faith in both mankind and nature and Art Nouveau went quickly out of style. If you are interested in owning some Art Nouveau furniture, lamps or objects of your own, though, you can find plenty of good examples of this style in antique shops throughout the United States.