Bars and clubs are scattered all over Santiago, but a handful of streets have such a concentration of establishments that they resemble block parties on Friday and Saturday nights. Pub crawls along Avenida Pío Nono and neighboring streets in Bellavista yield venues aimed at a young crowd (the drinking age is 18). Across the river and further west, Lastarria hosts a busy bar scene. To the east in Providencia, the area around Manuel Montt and Tobalaba metro stations attract a slightly older and better-heeled crowd.
What you should wear depends on your destination. Bellavista has a mix of styles ranging from blue jeans to basic black and, in general, the dress gets smarter the farther east you move, but remains casual.
Note that establishments referred to as "nightclubs" are almost always female strip shows. The signs in the windows usually make it quite clear what goes on inside. The same is true for certain cafés with blacked-out windows, called "cafés con piernas" (literally: coffee with legs).