This is a photo of a Paris Métro or Métropolitain entrance, with Art Nouveau style. The name Hector Guimard is almost synonymous with Art nouveau in Paris. The first encounter of Art nouveau for many travelers takes place at the curvaceously flamboyant entrances to the Metro stations wrought in writhing green steel. The red lamps at the end of steel tendrils like orchid stalks announce rapid transportation to any corner of the city. The sign "Metropolitain" in the Guimard-designed typeface at the entrances is now ensconced into the legend of the city, as undissociable as the Eiffel Tower and the Sacre Coeur, other Parisian institutions that almost never survived their epoch. See very interesting website Europa 1900 - The Art Nouveau Style.
Of course the Paris Métro or Métropolitain is the rapid transit system in Paris. It is a symbol of the city, notable for its homogeneous architecture, influenced by Art Nouveau. It has 16 lines, mostly underground, and a total length of 214 km. There are 300 stations. Since some are on more than one line, there are 384 stops. Paris has the most closely spaced subway stations in the world, with 245 stations within the 41 square kilometres City of Paris. Paris is the second busiest metro system in Europe after Moscow. It carries more than 4.5 million passengers a day and Châtelet - Les Halles is the world's largest underground station.
The Métro is different from the RER (Réseau Express Régional, "Regional Express Network") which is another rapid transit system in France serving Paris and its suburbs.