Modifiers apply a transform to a representation of a scene object's state. An ordinary modifier is sometimes referred to as an object space modifier.
A modifier takes an object and either modifies it directly, or creates a new object from the input object. Many modifiers only apply to certain characteristics (i.e. channels) of an object such as the vertices or the faces.
Modifiers also may only operate on certain type of object. For example, the Edit Spline modifier only works on spline shapes (SplineShape) and the Volume Select Modifier only works on triangle mesh objects (TriObject).
In 3ds Max modifiers may be "instanced" which in the language of 3ds Max means that a single instance of a modifier is shared for multiple objects.