The "input splitter" object in Quartz Composer allows other applications to pass control information into your compositions.
For a step by step walkthrough, visit the following page on our wiki:
http://wiki.vidvox.com/index.php/Quartz ... shed_input
If you want to use a Quartz Composer patch as an effect or as a fontsynth patch, it has special requirements. Specifically, fx patches need to have an image-type input splitter published as "inputImage" (images that the fx is applied to will come through this input), and fontsynth patches need to have a string-type input splitter published as "FileInput" (letters, words, sentences, etc. will be passed through this input).