From Avisynth wiki
This section contains basic information about the AviSynth script language. It presents the fundamental properties and elements of the language and the ways that they can be combined to build an AviSynth script.
- The grammar of AviSynth script language. An introduction to AviSynth scripting.
- How to declare and use variables in your scripts.
- Available operators for each type of variable and their relative precedence.
- Functions that return a property of a clip.
- Language constructs for controlling the script flow.
- Ready-made non-clip functions to use in your scripts.
- How to define and use your own custom functions in AviSynth scripts.
- How to load plugins (AviSynth, VirtualDub, VFAPI and C-plugins). Plugin autoloading and name-precedence.
- Scripting on a per-frame basis.