Well.....heres a can of worms.......
As Bonzi says, its basically a choice of greater accuracy or greater distance. Graphite shafts provide greater distance because they are usually half the weight of steel (hence greater clubhead speed for the same effort swing) and because they are usually half an inch to an inch longer than steel to keep the swingweight the same (longer swing arc).
Steel are more accurate because they are zero torque (no graphite shaft yet made can come close for control) the extra weight also helps keep the shaft from twisting.
To be fair if you get a nicely weighted graphite shaft that you have to load up well you can get good accuracy out of it as well as distance, just as if you get a nice light steel shaft (True Temper TX for example) you can boost your distance.
Basically, I dont think I have seen a tour player use a steel shafted driver this millenium so go for graphite in the driver, I play steel in the fairway woods just because i like the feel of it for fairway woods. Try a few graphite shafts first, top makes are Grafalloy, Graphite Design, Aldila and UST.
Good luck,
D,