function OnGUI () {
    GUI.Button (Rect (0, 0, 100, 20), GUIContent ("A Button", "This is the tooltip"));
    // If the user hovers the mouse over the button, the global tooltip gets set
    GUI.Label (Rect (0, 40, 100, 40), GUI.tooltip);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnGUI() {
        GUI.Button(new Rect(0, 0, 100, 20), new GUIContent("A Button", "This is the tooltip"));
        GUI.Label(new Rect(0, 40, 100, 40), GUI.tooltip);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnGUI():
        GUI.Button(Rect(0, 0, 100, 20), GUIContent('A Button', 'This is the tooltip'))
        GUI.Label(Rect(0, 40, 100, 40), GUI.tooltip)