Scripting Reference
MonoBehaviour.OnMouseDown

function OnMouseDown () : void

Description

OnMouseDown is called when the user has pressed the mouse button while over the GUIElement or Collider.

This event is sent to all scripts of the Collider or GUIElement.

JavaScript 
// Loads the level named "SomeLevel" as a response
// to the user clicking on the object

function OnMouseDown () {
    Application.LoadLevel ("SomeLevel");
}
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnMouseDown() {
        Application.LoadLevel("SomeLevel");
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnMouseDown():
        Application.LoadLevel('SomeLevel')
This function is not called on objects that belong to Ignore Raycast layer.

OnMouseDown can be a co-routine, simply use the yield statement in the function. This event is sent to all scripts attached to the Collider or GUIElement.

IMPORTANT: This function has no effect on iPhone.