sun set = watching the sun as it sets in the sky for a period of time
     sunset = a single, snapshot-glimpse of a moment of the sun set

Late this afternoon, Laura and I went to the beach. It was a four-minute walk there and so we had a quick swim before dinner.

After we had cleaned up all of the dinner dishes, Louise, Laura, Emma and I went out for a walk. Half way through, we paused at a park to play and watch the sun set.