Input.GetKeyDown

Description

Returns true during the frame the user starts pressing down the key identified by name.

You need to call this function from the Update function, since the state gets reset each frame. It will not return true until the user has released the key and pressed it again.

For the list of key identifiers see Conventional Game Input. When dealing with input it is recommended to use Input.GetAxis and Input.GetButton instead since it allows end-users to configure the keys.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    void Update()
    {
        if (Input.GetKeyDown("space"))
        {
            print("space key was pressed");
        }
    }
}

Description

Returns true during the frame the user starts pressing down the key identified by the key KeyCode enum parameter.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    void Update()
    {
        if (Input.GetKeyDown(KeyCode.Space))
        {
            print("space key was pressed");
        }
    }
}