A grade of unleaded gasoline, usually with an (R+M)/2 method octane rating of 93. It usually costs 10-20% more than the 87 octane regular grade. This grade of gasoline is recommended for most sports cars, Volkswagens, luxury cars, and any other car with a high-compression engine. Also, running forced induction without premium gasoline is insanity.
Running this type offuel on a car that does not need it will not damage the car, however, it will not boost power or fuel economy either (you will just be wasting money buying a more expensive fuel). Running a lower grade of fuel on cars that need premium will only really harm cars without ECM systems (i.e. really old cars before the 80's). On cars with ECM systems, fuel economy and performance will suffer, but the car will still run without damage.