A linear foot is a simple measurement of length or distance, whereas a square foot is a measurement of area. To determine the number of square feet of an area or object, its height and width must be known.
Two objects 1 foot apart are at a distance of 1 linear foot from each other; a person who is 6 feet tall measures 6 linear feet.
The area of a square with sides each measuring 1 foot is 1 square foot (1 x 1). The area of a backyard measuring 25 feet wide and 40 feet long has an area of 1,000 square feet (25 x 40).