Derive from this class to create an editor window.

// JavaScript example:
class MyWindow extends EditorWindow {
	var myString = "Hello World";
	var groupEnabled = false;
	var myBool = true;
	var myFloat = 1.23;
	
	// Add menu named "My Window" to the Window menu
	@MenuItem ("Window/My Window")
	static function Init () {
		// Get existing open window or if none, make a new one:		
		var window = ScriptableObject.CreateInstance.<MyWindow>();
	}
	
	function OnGUI () {
		GUILayout.Label ("Base Settings", EditorStyles.boldLabel);
			myString = EditorGUILayout.TextField ("Text Field", myString);
		
		groupEnabled = EditorGUILayout.BeginToggleGroup ("Optional Settings", groupEnabled);
			myBool = EditorGUILayout.Toggle ("Toggle", myBool);
			myFloat = EditorGUILayout.Slider ("Slider", myFloat, -3, 3);
		EditorGUILayout.EndToggleGroup ();
	}
}

// C# example:
using UnityEngine;
using UnityEditor;
public class MyWindow : EditorWindow {
	string myString = "Hello World";
	bool groupEnabled;
	bool myBool = true;
	float myFloat = 1.23f;
	
	// Add menu named "My Window" to the Window menu
	[MenuItem ("Window/My Window")]
	static void Init () {
		// Get existing open window or if none, make a new one:
		MyWindow window = (MyWindow)EditorWindow.GetWindow (typeof (MyWindow));
	}
	
	void OnGUI () {
		GUILayout.Label ("Base Settings", EditorStyles.boldLabel);
			myString = EditorGUILayout.TextField ("Text Field", myString);
		
		groupEnabled = EditorGUILayout.BeginToggleGroup ("Optional Settings", groupEnabled);
			myBool = EditorGUILayout.Toggle ("Toggle", myBool);
			myFloat = EditorGUILayout.Slider ("Slider", myFloat, -3, 3);
		EditorGUILayout.EndToggleGroup ();
	}
}