From Wikipedia
Instant coffee is a beverage derived from brewed coffee beans. Through various manufacturing processes the coffee is dehydrated into the form of powder or granules. These can be rehydrated with hot water to provide a drink similar (though not identical) to conventional coffee. At least one brand of instant coffee is also available in concentrated liquid form.
http://en.wikipedia.org/wiki/Instant_coffee
Lower down it tells you the difference between freeze dried and spray dried