The wheel is the most essential part of the bicycle. There are three main parts of the wheel; the hub/rim, spokes and the tire.
T here are two main ways to spoke a wheel, radial and tangential. Radial spokes are arranged straight from the hub to the rim. Most wheels today are tangentially spoked which means that the spokes leave the hub at an angle. This allows for torque from the spokes to be turned into power.
Exploratorium Senior Scientist Paul Doherty says that "You can spoke the front wheel completely radially, but the rear wheel had better not be spoked radially. there is no way to convey the twist of the wheel out to the rim to drive you forward". Wheels that are radially spoked are also bettter equipped to handle the variety of forces acting on the wheel forces acting on the wheel. Contrary to what you might believe spokes are not used as a columnar supports, instead they work by pulling the hub and the rim together.
http://www.exploratorium.edu/cycling/wheel2.html