Input.GetButton

static function GetButton (buttonName : string) : bool

Description

Returns true while the virtual button identified by buttonName is held down.

Think auto fire - this will return true as long as the button is held down.

// Instantiates a projectile every 0.5 seconds,
// if the Fire1 button (default is Ctrl) is pressed.
var projectile : GameObject;
var fireRate = 0.5;
private var nextFire = 0.0;
function Update () {
if (Input.GetButton ("Fire1") && Time.time > nextFire) {
nextFire = Time.time + fireRate;
clone = Instantiate (projectile, transform.position, transform.rotation);
}
}

Use this only when implementing action like events eg. shooting a weapon. Use Input.GetAxis for any kind of movement behaviour.