The same correction over the entire lens. Examples would be readers, distance-only or computer glasses.
Correction for two different focal points. In other words, distance correction and reading correction.
Correction for three different focal lengths. Distance, intermediate and near correction.
Computer glasses, sunglasses, piano glasses are all examples of different lens styles for single vision.
Occupational, Flat-Top 28, 35, 45, and Round Seg are examples of different bifocals.
7X28, 8X35, 14X35 would be examples of different trifocals.