The IIR Gaussian Blur plug-in displaces pixels in a radius from a central point defined in the dialog. A higher value will produce a higher amount of blur. The blur can be set to act in one direction more than the other by clicking the Chain Button so that it is broken, and altering the radii.
By altering the ratio of horizontal to vertical blur, you can give the effect of a motion blur.
IIR blurring is most useful for images which are not computer generated such as photographs.