Dusk is the darkest stage of twilight in the evening. During early to intermediate stages of twilight, there may be enough light in the sky under clear-sky conditions to read outdoors without artificial illumination.
Civil dusk occurs when the earth rotates to a point at which the center of the sun is at 6° below the local horizon. This marks the end of the evening civil twilight, the point where artificial illumination is required to read outside.
Twilight comes after sunset, which is the point at which the earth has rotated just enough that the sun is no longer visible on the local horizon (under clear conditions).
The time at which the sun is 6 degrees below the horizon in the evening. At this time objects are distinguishable and some stars and planets are visible to the naked eye.
>Is when the sun is 12 degrees below the horizon in the evening. At this time, objects are no longer distinguishable, and the horizon is no longer visible to the naked eye.
The time at which the sun is 18 degrees below the horizon in the evening. At this time the sun no longer illuminates the sky, and thus no longer interferes with astronomical observations.
— derived from Wikipedia