The above image compares three related painting techniques that often get confused: washing, scumbling and glazing.
What is a paint wash?
Put simply, wash painting is a hue or tint applied in a thin transparent layer.
Color wash painting
Washes should be applied loosely in the very beginning stages of painting. If you are worried about the wash blurring your lines, fixative can help set the drawing.
See also: How to make a pastel wash; How to wash over drawings