MonoBehaviour.Start()

Description

Start is called on the frame when a script is enabled just before any of the Update methods is called the first time.

Like the Awake function, Start is called exactly once in the lifetime of the script. However, Awake is called when the script object is initialised, regardless of whether or not the script is enabled. Start may not be called on the same frame as Awake if the script is not enabled at initialisation time.

The Awake function is called on all objects in the scene before any object's Start function is called. This fact is useful in cases where object A's initialisation code needs to rely on object B's already being initialised; B's initialisation should be done in Awake while A's should be done in Start.

Where objects are instantiated during gameplay, their Awake function will naturally be called after the Start functions of scene objects have already completed.

using UnityEngine;
using System.Collections;

// The ExampleClass starts with Awake.  The GameObject class has activeSelf
// set to false.  When activeSelf is set to true the Start() and Update()
// functions will be called causing the ExampleClass to run.
// Note that ExampleClass (Script) in the Inspector is turned off.  It
// needs to be ticked to make script call Start.

public class ExampleClass : MonoBehaviour
{
    private float update;

    void Awake()
    {
        Debug.Log("Awake");
        update = 0.0f;
    }

    IEnumerator Start()
    {
        Debug.Log("Start1");
        yield return new WaitForSeconds(2.5f);
        Debug.Log("Start2");
    }

    void Update()
    {
        update += Time.deltaTime;
        if (update > 1.0f)
        {
            update = 0.0f;
            Debug.Log("Update");
        }
    }
}