AviSynth is a very powerful tool for video post-production. It provides almost unlimited ways of editing and processing videos. AviSynth works as a frameserver, providing instant and very fast editing without the need for temporary files.
AviSynth itself does not provide a graphical user interface (GUI) but instead relies on a script system that allows advanced non-linear editing. While this may at first seem tedious and unintuitive, it is remarkably powerful and is a very good way to manage projects in a precise, consistent, and reproducible manner.

Because text-based scripts are human readable, projects are inherently self-documenting. The scripting language is simple yet powerful, and complex filters can be created from basic operations to develop a sophisticated palette of useful and unique effects.