A filter service is a piece of software that can transparently translate a file format into one of the standard NEXTSTEP formats, such as:
If you are using OmniWeb as your browser, chances are you may have installed in your computer OmniImageFilter.service. OmniImageFilter.service is a good example of a filter service provider. OmniImageFilter.service will automatically translate about 33 different foreign graphic formats. With this filter installed, you could use any of the file types supported by the filter as if they were TIFF images, a supported NEXTSTEP graphic file type format.