The waterfall is a style of race start generally used in longer races in track and field, with few (usually less than 20) competitors. If there are more racers, a barrel start is used.
At the starting line of the track, there is a curved lined that starts at the start/finish line of the inside lane, and curves out to a few yards ahead of the start/finish line in the outer lane. In the start, racers line up a few steps behind this curved line, and use it as their starting line. At the call of "runner's set," they take two steps forward, take set positions, and wait for the gun, with their toes behind the line.
The advantages of the waterfall are a simple race. Everybody starts on the same line, and cuts in to the inside lane when they can. However, this can lead to some rough starts (elbows, spikings, etc.) It is generally more successful in longer races such as the 3000, 5000, 1600 and 1500, although a staggered start is sometimes used in the 1500 and 1600.