Art Nouveau was an international style popular from 1890-1910 that featured sinuous and flowing organic lines inspired by nature. It emerged in various visual arts including architecture, where some key features were asymmetrical shapes, arched forms, and plant-like embellishments. Important Art Nouveau buildings included Casa Batlló and Casa Milà in Barcelona, Spain, known for their curving forms and use of materials like mosaic tiles and stained glass, as well as the Horta Museum in Belgium, featuring exposed columns, curved glass, and continuity of curved designs.