I concur precisely with Rudi.

joergi, tip and gratuity are the same. Gratuity is just a more formal or classy way of stating it.

In the USA if the check says "Service Charge of xx% Included" it usually refers to the tip. Many restaurants use this concept for parties of about 8 or more to ensure an appropriate gratuity for their wait staff. In many hotels, it is also used on room service orders of any size.