Scripts and plugins are extensions to Art of Illusion. The usual questions someone asks when confronted to extensions to a software he knows little about are:
-Do I need any?
-If yes, which ones?
First let us reassure you: scripts and plugins are loaded only when they're used. You can safely install the whole of them without any downside.
This being said, what are exactly scripts and plugins? Scripts are generally small portions of code written in a scripting language pretty close to Java (beanshell). They are interpreted "on the fly" when evaluated. As for plugins, they are delivered as compiled Java code exactly like Art of Illusion itself. Since compiled code is much faster than interpreted code, scripts are used to perform specific limited tasks, like for example converting an object to its mirror opposite. Plugins, on the other hand, are generally used to bring new features to AoI: a preview renderer, more import/export file formats, a polygonal mesh modeler, etc.
There are different flavors of scripts. The first type of scripts are Tool scripts. These scripts are run once to modify the scene. Basically they add objects or modify existing ones. Once they're run, the scene is permanently changed.
The second type of scripts are Object scripts or Scripted Objects. These scripts are executed before the scene is rendered. Once the scene is rendered, the effects of the scripts are forgotten -until the scene is rendered again and the script is run-. These scripts can be used to change the aspect of an object versus time, for example. Or to add hair to the surface of mesh. Simple particle systems can even be obtained using such scripts.
The third type of scripts are Startup scripts. As the name implies, these scripts are executed at AoI startup. There aren't many of them and you can safely ignore them for the moment.
Perhaps the script that has the most general scope is the Text script. It allows to import text as 3D objects in the scene, which is an important feature. Other scripts target special cases or situations, they are invaluable when you find yourself in those situations for which those scripts have been written.
The features brought by plugins vary greatly in nature and importance. Some bring new user interface looks (Theme plugins), some make it possible to connect to IRC AoI chat room where you'll meet experienced users most likeley to bring you the help you need. But if you really want t know what the most important plugins are, those which bring new features to the program itself, here they are (list may be outdated):
-The Advanced Renderer. Your swiss army knife for rendering, except you can live without a swiss army knife.
-The PolyMesh plugin. It brings polygonal meshes to AoI and hence box modeling. You can also draw the mesh from reference pictures. It features its own smoothing method and allows to unfold meshes for UV mapping purpose.
-The TelefishAoI plugin lists many more file formats to import from/export to than the default .obj format that AoI features.