Student nurses working in a clinical setting usually wear a uniform. The uniform is worn for hygiene as well as identification purposes.
The women in this photograph are wearing a uniform consisting of a dress, pinafore apron and cap. There are however variations, and in some hospitals, student nurses also wear a nursing pin, or the pinafore apron may have been replaced by a cobbler style apron. A "scrub dress" is a simpler type of uniform, sometimes worn in operating rooms.
However, student nurses are more likely to wear practical and durable "scrubs", eschewing the old-fashioned cap and ruffles these days. It has been argued that the wearing of uniforms by nurses is unnecessary in terms of hygeine, for example, and may be more to do with a medically-dominated, hierachical system in hospital settings, and the military roots of nursing.
See: candystriper, Student uniform