Project Folders

Here is a list of the various standard/recommended project folders in use. These are folders which appear inside your Project window, therefore effectively within your Assets folder on your filesystem.

Unless otherwise stated these are all relevant to Unity 3.4.

Folder name	Description	Folder location
Resources	Any resources you wish to load using Resources.Load must go here	Anywhere
Editor	Any scripts which alter the Unity editing environment itself must go in here	Anywhere, or "Standard Assets/Editor", "Pro Standard Assets/Editor", "Plugins/Editor" if in one of these special folders
Plugins	Any external DLLs and scripts go in here (Pro only), see: Manual: Plugins	In project root only
Standard Assets	Standard scripts & other assets go in here when imported (i.e. when creating a new project)	In project root only
Pro Standard Assets	As above but for Pro users only	In project root only
Gizmos	Scripts which are used to display gizmos in the editor - excluded from any build	In project root only
WebplayerTemplates	Scripts in this folder will not be compiled	In project root only
Editor Default Resources	Resources in this folder are available to EditorGUIUtility.Load(). A good place to keep GUISkins. [1]	In project root only
StreamingAssets	Files in this folder will be included verbatim in the built project, see: Manual: Streaming Assets	In project root only

References

1. http://answers.unity3d.com/questions/29606/can-you-apply-a-guiskin-to-an-editor-gui.html