// Most of the time you can use:
// transform.LookAt instead

var target : Transform;
function Update () {
var relativePos = target.position - transform.position;
var rotation = Quaternion.LookRotation(relativePos);
transform.rotation = rotation;
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
public Transform target;
void Update() {
Vector3 relativePos = target.position - transform.position;
Quaternion rotation = Quaternion.LookRotation(relativePos);
transform.rotation = rotation;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

public target as Transform

def Update():
relativePos as Vector3 = (target.position - transform.position)
rotation as Quaternion = Quaternion.LookRotation(relativePos)
transform.rotation = rotation