GUILayout.BeginScrollView  

// The variable to control where the scrollview 'looks' into its child elements.
var scrollPosition : Vector2;

// The string to display inside the scrollview. 2 buttons below add & clear this string.
var longString = "This is a long-ish string";

function OnGUI () {    
    // Begin a scroll view. All rects are calculated automatically - 
    // it will use up any available screen space and make sure contents flow correctly.
    // This is kept small with the last two parameters to force scrollbars to appear.
    scrollPosition = GUILayout.BeginScrollView (
        scrollPosition, GUILayout.Width (100), GUILayout.Height (100));
    
    // We just add a single label to go inside the scroll view. Note how the
    // scrollbars will work correctly with wordwrap.
    GUILayout.Label (longString);
    
    // Add a button to clear the string. This is inside the scroll area, so it
    // will be scrolled as well. Note how the button becomes narrower to make room
    // for the vertical scrollbar
    if (GUILayout.Button ("Clear"))
        longString = "";
    
    // End the scrollview we began above.
    GUILayout.EndScrollView ();
    
    // Now we add a button outside the scrollview - this will be shown below
    // the scrolling area.
    if (GUILayout.Button ("Add More Text"))
        longString += "\nHere is another line";
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Vector2 scrollPosition;
    public string longString = "This is a long-ish string";
    void OnGUI() {
        scrollPosition = GUILayout.BeginScrollView(scrollPosition, GUILayout.Width(100), GUILayout.Height(100));
        GUILayout.Label(longString);
        if (GUILayout.Button("Clear"))
            longString = "";
        
        GUILayout.EndScrollView();
        if (GUILayout.Button("Add More Text"))
            longString += "\nHere is another line";
        
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public scrollPosition as Vector2

    public longString as string = 'This is a long-ish string'

    def OnGUI():
        scrollPosition = GUILayout.BeginScrollView(scrollPosition, GUILayout.Width(100), GUILayout.Height(100))
        GUILayout.Label(longString)
        if GUILayout.Button('Clear'):
            longString = ''
        GUILayout.EndScrollView()
        if GUILayout.Button('Add More Text'):
            longString += '\nHere is another line'