Scripting Reference
Debug.Log

static function Log (message : object) : void

Description

Logs message to the Unity Console.

JavaScript 
Debug.Log ("Hello");
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        Debug.Log("Hello");
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        Debug.Log('Hello')

static function Log (message : object, context : Object) : void

Description

Logs message to the Unity Console.

When you select the message in the console a connection to the context object will be drawn. This is very useful if you want know on which object an error occurs.

JavaScript 
Debug.Log ("Hello", gameObject);
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        Debug.Log("Hello", gameObject);
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        Debug.Log('Hello', gameObject)