Linear Feet: a straight line. Example, a straight line from point A to point B is the linear distance. If you have a fence that is 6 feet high and 100 feet long then your fence is 100 linear feet long (the width is not taken into consideration). Wood fence, chain link fence, aluminum fence, vinyl fence and chain link privacy slats are often sold by the linear foot.
Square Foot: 12 inches by 12 inches is equal to one square foot. To calculate a square or rectangle just multiply the height by the width. Wind screening is often sold by the square foot. If your fence is 6 feet high and 100 feet long then your fence is 600 square feet.