The most basic difference between a CAV unit and a VAV unit is the way by which they modulate air.
A constant volume system (CAV) can only produce air at a constant volume. To condition a space, the CAV produces air at a constant volume but changes the temperature to achieve space temperature setpoints.
A variable system (VAV) can produce air at varying volumes. Instead of changing the temperature in the system like a CAV, the VAV changes the volume of air being pushed to a space at a constant temperature.
Example
An air handling unit (AHU) may have a VAV terminal unit installed before one of the spaces being served. The supply air temperature leaving the AHU can be at 55°F, but the VAV terminal unit (or box) can use a damper to vary the volume of air coming through it and into the space. If the space temperature setpoint is set to be warmer, then the damper can be fully closed. Opening the damper more will allow more cool air from the AHU to enter the space, and lower its temperature.