Frameserving is a process by which video data is transferred from one program to another. No intermediate or temporary files are created. The program that opens the source file(s) and outputs the video data is called the frameserver. The program that receives the data could be any type of video application. The input looks like a relatively small, uncompressed video file. This feature of frameserving enables you to open certain types of files in an application that wouldn’t normally support them.
There are three programs that are commonly used as frameservers. Those programs are Avisynth, VirtualDub, and VFAPI.
Further informations about frameserving are available at the AviSynth wiki-website.