// 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