SOG = sea of green
This is where many small plants are used and forced to flower at an earlier age, mainly with the hope of harvesting the top cola of a plant. The canopy of the plants is lush, creating the vision of a Sea of Green.
ScrOG = Screen of Green
This is where fewer plants are used, and screen (usually chicken wire with 2" holes) is placed above the plants at a fixed height. When the plants reach the screen, they are trained to grow where the cultivator desires. This will allow a thicker canopy, and no empty spaces on the screen so that more surface area of the plant is reached by the light, creating the possibility of secondary, tertiary etc. growth. The screen is effectively full with the plant's trained canopy, creating the vision of a Screen Of Green.