- light_environment defines (a) the color and intensity of the Diffuse Skylight and (b) the direction, color and intensity of the Direct Sunlight.
- It also creates (buggy, unconfigurable) Dynamic Shadows which can only be overridden/controlled by the shadow_control Entity.
- env_sun places a glow sprite in the skybox to represent the Sun's apparent position in the sky.
To make sense of it all, just make sure you have only one of each of these entities in your map, and that the Pitch and Yaw <angles> are set the same for all three of them. (Pitch is equivalent to the Sun's Angle of Elevation from the ground, and Yaw is its Compass Bearing.)
The Sky List article suggests some settings for some of Valve's skyboxes. The worldspawn entity defines which skybox to use. Note also that env_fog_controller settings will look wrong if they don't correspond to the particular skybox in use.