A style of architecture and decorative art similar to art nouveau, popular in German-speaking areas of Europe during the late 19th and early 20th centuries. There is some in Amsterdam, but not much. The name comes from Munich magazine Die Jugend (“Youth”), founded in 1896, which featured Art Nouveau designs. It is primarily a style in architecture characterised by swirling lines, glazed brickwork and ornate tiling. Austrian painter Gustav Klimt is said by some to be Jugenstil.
One link I found said Jugendstil grew out of an innate desire for doodling. I can understand that.
The city's listed building organisation Amterdam Heritage says these are Jugendstil buildings:
Keizersgracht 766 (1894)
Helios, Spui 15-19 (1895/96)
Haarlemmerdijk 39 (1896)
Raadhuisstraat 23-55, Winkelgalerij (1896/99)
Rokin 58 (1898)
Café-restaurant De Kroon, Rembrandtplein 17 (1898)
Spuistraat 274 (1898)
Witte Huis, Raadhuisstraat 2-6 (1899/1901)
Algemeen Handelsblad, Nieuwezijds Voorburgwal 234-240 (1902/03)
Damrak 37 (1903)
Athenaeum boekhandel, Spui 14-16 (1904)
Rokin 69 (1901)
Raadhuisstraat 52-54 (1902/03)
Damrak 80-81 (1903/04)
Keizersgracht 174-176 (1904/05, uitgebreid 1968/69)
Haarlemmerstraat 83 (1906)
This link to Amsterdam Heritage's website, also only in Dutch so far, has photos of some of these buildings. Alas, it is all in Dutch. One day I will get round to putting them all into English!