Dry ice is frozen carbon dioxide (CO2). This name was once a trademark of Prest Air Devices (in 1925), but is now a common term for any such product. It is useful for a variety of purposes, but is especially popular for temporary refrigeration to preserve food, organs, or other organic matter. It is also used in some scientific research.
While water freezes at a temperature of 32° Fahrenheit (or 0° Celsius), carbon dioxide freezes at -109.3° Fahrenheit (or -78.5° Celsius), which means that while harder to create, dry ice can also chill or freeze things much better than regular ice. Another significant advantage of dry ice is that it undergoes sublimation as it melts--it converts directly from a solid to a gas. This means that it does not wet the item(s) it is cooling, but simply "disappears" as a gas into the environment. It is for this reason that is has gotten the name, "dry ice," because it remains dry as it melts.