I take it that those of you who dont understand what a Prerunner is dont follow off-road racing.
Quite simply, in off-road racing such as Baja, a prerunner is a truck that they use to practice or, "pre run" the race course. Its very similar to the actual race truck, but its a 4x2 and the engine isnt quite as hopped-up as what the race truck has. This allows them to practice the course and tear up the truck without putting unneccesary wear and tear on the race truck.
The was the inspiration for the Tacoma Prerunner. Its a truck that is identical to the 4x4 in terms of frame and suspenion. The only difference is that it lacks the front running gear (transfer case, differential, driveshaft and CV joints) that a 4x4 has. Other than that, there is no different (from a mechanical standpoint) from a 4x4 to a Prerunner.
The Prerunner appeals to people who like the tall stance of a 4x4, but whom dont really need a 4x4. A Prerunner is, on average, about $5,000 less expensive to buy than a 4x4. Also, because it lacks the front running gear of a 4x4, in theory it should be more reliable than a 4x4 in the long run (fewer moving parts = better reliability).