Bifocals have a line separating an upper, distance prescription from a lower, near prescription. Since we as human beings hold things in our hands when we want to see stuff "up-close", and since our hands are below our eyes, the near segment is almost always placed in the lower half of the eyeglass frames. The Optometrist, or their staff, makes specific measurements and places the bifocal line in the position that is the most convenient for the patient.
Progressive lenses have a "seamless" or "invisible" transition between the two prescription "zones" but accomplish the same purpose as a bifocal. They still have an upper, distance prescription and a lower near prescription, but apart from these, progressive lenses can also be designed to give the patient a useful intermediate prescription. This offers progressive lens wearers an optical advantage apart from the cosmetic advantage of having no lines on their eyeglasses. You can learn more about progressive lens design and fabrication here.