Quaternion.LookRotation

Parameters

Description

Creates a rotation with the specified forward and upwards directions.

Z axis will be aligned with forward, X axis aligned with cross product between forward and upwards, and Y axis aligned with cross product between Z and X.

Returns identity if the magnitude of forward is zero.
If forward and upwards are colinear, or if the magnitude of upwards is zero, the result is the same as Quaternion.FromToRotation with fromDirection set to the positive Z-axis (0, 0, 1) and toDirection set to the normalized forward direction.

// You can also use transform.LookAt

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    public Transform target;

    void Update()
    {
        Vector3 relativePos = target.position - transform.position;

        // the second argument, upwards, defaults to Vector3.up
        Quaternion rotation = Quaternion.LookRotation(relativePos, Vector3.up);
        transform.rotation = rotation;
    }
}

Additional resources: SetLookRotation.