A colorspace which describes any color by it's hue, saturation, and luminance. You can visualize an HSL color space as a cylindrical coordinate system. Hue is an angle about the axis of the cylinder, luminance is a distance along the radius of the cylinder, and saturation is a distance along the axis of the cylinder.

Most computer graphics programs that use the HSL color space usually "unroll" the cylinder into a square with hue across the horizonal axis, saturation along the vertical axis, and luminance on a seperate slider. The GIMP uses this type of color selection. Some graphics packages display hue as a circle, with a triangle of saturation and luminance rotating inside it. (this is hard to describe, look at fractal design painter, or devel versions of The GIMP to see this).