Something that's fashionable is in style. Your cousin might insist that wearing wide-brimmed fishing hats is very fashionable right now.
Things that are very trendy or popular are fashionable, whether they're a brand of jeans or a particular baked good that everyone seems to be eating. A person who follows these trends can also be described as fashionable: "Did you see John walk by wearing those yellow boots and eating a huge doughnut? He's so fashionable." The noun fashion is at the root of fashionable, with its Old French origin, façon, "face, appearance, design, or beauty."