Simple moving Cam in direction its facing

 var moveSpeed  : float = 20.0;
 function Update(){
 if(Input.GetKey(KeyCode.W)){
    transform.localPosition.z += moveSpeed * Time.deltaTime;
 }
 if(Input.GetKey(KeyCode.S)){
    transform.localPosition.z -= moveSpeed * Time.deltaTime;
 }
 }

The Previous answer does not account for the camera's pitch (rotation around the x vector) which is common in games (top down camera, free rotate camera, over-the-shoulder third person camera, etc). In order to account for this, use the following code.

  private void Update()
     {
         Vector3 movement = Vector3.zero;
         Vector3 camEuler = Camera.rotation.eulerAngles;
         camEuler.x = 0f;
         Quaternion normalizedRotation = Quaternion.Euler(camEuler);
 
         if (Input.GetKey(KeyCode.W))
         {
             movement += normalizedRotation * Vector3.forward;
         }
 
         if (Input.GetKey(KeyCode.S))
         {
             movement -= normalizedRotation * Vector3.forward;
         }
 
         if (Input.GetKey(KeyCode.A))
         {
             movement -= normalizedRotation * Vector3.right;
         }
 
         if (Input.GetKey(KeyCode.D))
         {
             movement += normalizedRotation * Vector3.right;
         }
 
         if (movement.magnitude > 1f)
         {
             movement = movement.normalized;
         }
 
         _charController.Move(movement * WalkSpeed * Time.deltaTime);
     }

This snippet removes the pitch from the camera's current rotation (zeroing out the x portion of the camera's euler angle), making it level with the world. To transform this into a usable movement vector, multiply the resulting rotation (normalizedRotation) by whichever direction you want to move the object.