linear ft is the length of something, such as a backsplash, with no regard to width. for example a backsplash 8 feet long and 4 inches high is 8 linear feet. if it is 8 feet long and 6 inches high it is still 8 linear feet.
square inches or square feet refers to area, which is length times width. in the above examples, 8 ft long times 4 inches high( 1/3 ft)equals 2 and 2/3 sq ft; 8 ft long times 6 inches high equals 4 sq ft