Scripting Reference
Quaternion.FromToRotation

static function FromToRotation (fromDirection : Vector3, toDirection : Vector3) : Quaternion

Description

Creates a rotation which rotates from fromDirection to toDirection.

Usually you use this to rotate a transform so that one of its axes eg. the y-axis - follows a target direction toDirection in world space.

JavaScript 
// Sets the rotation so that the transform's y-axis goes along the z-axis

transform.rotation = Quaternion.FromToRotation (Vector3.up, transform.forward);
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        transform.rotation = Quaternion.FromToRotation(Vector3.up, transform.forward);
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        transform.rotation = Quaternion.FromToRotation(Vector3.up, transform.forward)