If a drink is served "on the rocks," what is it served with?

While the phrase "on the rocks" is used frequently at the bar and in popular culture, it means one very simple thing — with ice. The "rocks" are just cubes of ice. This is as opposed to another phrase that you have probably heard before: "neat". When you order a whiskey "neat", what you are ordering is a glass of whiskey with no ice and no mixers. Any drink that you order at the bar can be ordered on the rocks to ensure that there will be ice in your cup.

Source: Oak & Eden