How to create Prefabs in Unity3D?

This short article explains what Unity's Prefabs are and what they are useful for.

Motivation

Let's imagine that we want to make a Tower Defence game in Unity. At first we would create a Tower GameObject, that would look something like this in the Hierarchy:

When the player starts the game, usually there are no Towers in the scene yet. What we need is a way to save our Tower GameObjects somewhere and then load them into the Hierarchy again as soon as the player wants to build a tower.

This is what Prefabs are for.

Creating a Prefab
Let's create a Prefab. It's very easy, all we have to do is drag our GameObject from the Hierarchy into the Project area like this:

Afterwards we can see the Prefab in our Project area:

Now we can delete the GameObject from our Hierarchy so it's not in the game world anymore. But since it's in the Project area, we don't lose it completely.

Loading a Prefab
If we want to load a prefab, we have two options. We can either do it manually by dragging it from the Project area into the Hierarchy area, or by using a script that calls Instantiate.

Here is an example script that loads a Prefab as soon as the game starts:

using UnityEngine;
using System.Collections;

public class Test : MonoBehaviour {
public GameObject prefab = null;

void Start () {
    // instantiate the prefab
    // -> transform.position means that it will be
    //    instantiated at the position of *this*
    //    gameobject
    // -> quaternion.identity means that it will
    //    have the default rotation, hence its not
    //    rotated in any weird way
    Instantiate(prefab,
                transform.position,
                Quaternion.identity);
}

}

Now all we have to do is add the Test script to a GameObject in the scene, take a look at the Inspector and then set the public prefab variable to any Prefab from our Project area like this:

After pressing start, it immediately loads our Tower Prefab into the scene.

Simple as that!