A damper, in HVAC terminology, is an air valve used to regulate the flow of air through the duct.
There are two general kinds of dampers (definitions are probably not in line with HVAC terminology, but hopefully understandable enough):
- Simple, also known as bang-bang, dampers - these can only be in fully open or fully closed position;
- Modulating dampers are able to stay in any position between fully open and fully closed.
First kind is, of course, cheaper, but has serious drawbacks - first, the noise they make is quite noticeable (one can get used to it, though), second, they provide only limited control and are not suitable to advanced applications.
Modulating dampers, on the other hand, are silent by design and provide adequate control, but require more sophisticated control systems in order to utilize the capabilities.