Well in fact, river racing is also known as wild water racing, wacky racing, wildwater canoeing and down river racing.
They all refer to the same thing, which has been given a little intro already. River Racing involves going down a river on timed runs in as fast a possible time as you can, without stopping.
A Sprint race has two short runs where the times are added together. The Classic race is a single run, of a longer distance - usually 15-20 minutes in length.