Single vision lenses have one single prescription across their entire surface and are suitable in all scenarios, whether you need your glasses for driving, working or reading. These lenses have the same optical power (the degree to which the lens converges or diverges the light) throughout. Hence, they can be used to correct short-sightedness, long-sightedness, astigmatism, or a combination of these.
A bifocal lens provides distance and near vision in one lens. The near vision correction of the lens is contained in a segment, often visible onthe front of the lens. This segment provides the extra power necessary for you to read clearly. Hence, theupper part tends to be used for seeing things at a distance and the lower part for seeing things close up, for example when you are reading.