What's the difference between the upper and lower parts of a bifocal lens?

Far v. near focus. A classic bifocal (with a line) has an upper portion focused on distance and a lower portion focused on reading and other near things. A proper eyeglass prescription will allow these two distances to be in focus when the natural lens of the eye stiffens over age 40 in a condition termed presbyopia. Some bifocals (lineless or progressive) have a continuous grind and no visible line.