Individual vents are vents that serve only one fixture, where dual vents serve two fixtures, connected with a double fitting. Individual vents are commonly used to vent fixtures that may be too far from a branch or stack to be wet vented, or for fixtures tied into a soil or waste pipe downstream of a water closet.
To make things just a little more complicated, if the vent for an individual or dual vent extends from a vertical soil or waste pipe(like they do in the images below), it can also be called a continuous vent but will then follow different rules and sizing regulations.
The size of individual and dual vents are determined by the size of the trap(s) only; the length does not matter. For residential applications you will be safe using 1-1/2″ pipe for either of these vents to serve traps up to 4″ in size. *Check with your code authority.