The blur filters are used to smoothen out the hard edges and details in the images. The resulting image is blurry. below is an example of a blurred image. The image of Kiki on right is the result of blur filter applied to the image on left.
There are many different filters for blurring:
You can input the horizontal and vertical radius for the amount of blurring here.
Doesn't only blur, but also subtly smudge an image into a direction of the specified angle thus giving a feel of motion to the image. This filter is often used to create effects of fast moving objects.
This filter creates a regular blur.
Lens Blur Algorithm.