Bifocals were the first way that lenses contain two or more lens that has multiple powers to help you see objects at all distances after you lose the ability to naturally change the focus of your eyes due to age (presbyopia)

Many people still prefer the line bifocal and trifocals because they have a hard time accomodating to the no line progressive lenses and experience nausea due to it.

Line bifocals are different from progressives because not only you see the added power on the lens, but you have your distance prescription on the entire area of the lens.

You can also have a trifocal lens where the flat top at the bottom has not only your near prescription but your intermediate. The intermediate portion is for distances such as the computer screen or the dash on your car.