The Motion Blur filter is a filter that will "push" the pixels of your image for you, creating a movement blur. The filter is capable of Linear, Radial, and Zoom movements. Each of these movements can be further adjusted, with Length, and Angle settings available.
Linear motion is a blur that travels in a single direction, horizontally, for example. In this case, Length will affect the perceived distance that the pixels are moved from their original positions. More Length will result in more blurring. Angle describes the actual angle of the movement. Thus, a setting of 90 will produce a vertical blur, and a setting of 0 will produce a horizontal blur.
Radial motion blur creates a circular blur. The Length slider is not important with this type of blur. Angle on the other hand, is the primary setting that will affect the blur. More Angle will result in more blurring in a circular direction. The Radial motion blur is similar to the effect of a spinning object. The center of the spin in this case, is the center of the image.
Zoom blurring produces a blur that radiates out from the center of the image. The center of the image remains relatively calm, whilst the outer areas become blurred toward the center. This filter option produces a perceived forward movement, into the image. Length is the main setting here, and affects the amount of speed, as it were, toward the center of the image.