In the U.S. military, battle stars are metallic devices that attach to the individual decorations called campaign medals or ribbons. The medal, sometimes derisively called "I was there", simply show that a person had served in a given area of warfare or potential warfare. The battle star shows that the wearer actually participated in battle in that area.
The term battle star is sometimes used for recognition that a unit served in combat, but the more common heraldic way of designating such history is attaching a battle streamer to the unit flag. Still, it is not uncommon to speak of warships having received battle stars.