Ambient lighting is generally created with ceiling fixtures like these can-lights. Ambient gives overall light to a room.
Task lighting is used to help light a task, like cooking, reading, sewing or homework.
Accent lighting is used to highlight special features in any space, architectural elements, wall art and special decorative items. These sconces for example, highlight the specialized wall treatment.
Decorative lighting usually serves more than one purpose. This table lamp is both decorative and a task light.
Source: Ginger Foust, IRIS