From the end user's point of view generally there is no big difference between a filter and a style. Both can be applied to an image and both have parameters to fine tune the result.
In PostworkShop terminology filters are more or less simple image processing algorithms coded by the software developers and you can't change them, except by setting the parameters to different values.
PWS filters can also provide results that would never exist in a Photoshop filter as these results are not images but e.g. a collection of directions to determine how to draw brush strokes by another filter.
Styles are more complex (sometimes very complex) image processing effects that you, the user can create by combining the filters and other styles together. Most of the built in effects are styles created by artists and not developers.
A style can be considered as something similar to a Photoshop action, but it is much more powerful.
Why to use PostworkShop instead of Photoshop filters?
If you like to create your own effects, you have unlimited possibilities to experiment and even create better styles than the built in ones.
If you just use the available styles, then you get 300+ built-in styles (for the price of a single Photoshop filter collection having only a limited scope) and a growing collection of free and commercial styles to download.
In PostworkShop all the styles are presented in an easy to browse categorized collection with thumbnails. If you already have your favorite style, applying it to a new image is as simple as a double-click.
On the other hand, if you are an experimenting type of artist, in PostworkShop you have very sophisticated means to create your final images. Imagine a Photoshop where layers can contain other layers containing a collection of filters connected in chained and parallel image processing pipelines. But of course this is not mandatory. Your first image is just a double-click away.
but not least, the PostworkShop Team writes
dedicated artistic image manipulation
filters that you won't find anywhere else.
Using these filters you or other artists can
create even more exciting styles that you
will have even more difficulties to find