1. Single vision lens means -the lens has only one power; Bifocal - two powers; Multifocal or progressive is self explanatory.
2. Bifocals/Multifocals are prescribed to give good vision for long as well as short/near .
3. Single vision lens can be made for presbyopia, if there is no power for long distance. The inconvenience is that you have to remove spectacles every time you see a distant object.
4. Sometimes seperate glasses are prescribed for distance and near if someone is not able to adjust to bifocals.