How to make CustomEditor as in Terrain script ?

Is is possible to make a CustomEditor script that has the appearence of the Terrain script ?
I am especially interessed in the field which shows the tree object thumbnails and seems to work like a list of GameObjects.

Thanks in advance for any reply.

this tutorial will help you start it
http://unity3d.com/learn/tutorials/modules/intermediate/editor/building-custom-inspector

now you have to realize that a lot of work goes into a custom inspector as complicated as that.
This is the API for the EditorGUILayout(its what i use)

just plan out before you start, saves time later