OnGUI is called for rendering and handling GUI events.

This means that your OnGUI implementation might be called several times per frame (one call per event). For more information on GUI events see the Event reference. If the MonoBehaviour's enabled property is set to false, OnGUI() will not be called.
	function OnGUI () {
		if (GUI.Button (Rect (10,10,150,100), "I am a button"))
			print ("You clicked the button!");
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnGUI() {
        if (GUI.Button(new Rect(10, 10, 150, 100), "I am a button"))
            print("You clicked the button!");
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnGUI() as void:
		if GUI.Button(Rect(10, 10, 150, 100), 'I am a button'):
			print('You clicked the button!')

For more information, see the GUI Scripting Guide.