Photoshop Filters

Originally similar to camera lens filters
Photoshop filters do much, much more.
Filters work in different ways:
Some analyze each pixel in the image or selection and transform it via mathematical algorithms to create random or predefined shapes.
Others sample groups of pixels and make transformations based on color differences, etc.
Filters can be applied to individual channels. Sometimes a good way to sharpen an image is to sharpen one channel.

Need to be in RGB or Grayscale mode to apply filters
Select the area to which you want to apply
copy selection into its own layer (Layer>new layer via copy or Ctrl+J)
apply the filter to the layer
if you don't like it, drag it into the trash
Check preview box to see what filter effect will be -- works for some of filters.
You can lessen filter effects by "fading" out filter effect once you have applied it (Filter/Fade command only available right AFTER filter applied)
Fade filter effect by applying filter at less than 100%.
Can always "undo" filter effects, delete step from history palette, or reapply them