A linear foot is the same measurement as a foot. The linear foot measures 12 inches in length. Linear is used to describe the total length of an item without regard to width or thickness.
Simply measuring items by the standard 12 inch foot is the same measurement. Linear means a straight line, and indicates attention to the length dimension only. Lumber sold by the linear foot simply indicates the length of the boards placed end-to-end, no matter how wide or thick they are. Similarly, a bolt of cloth is sold by the linear foot, regardless of the width of the fabric.