There are a number of factors that prevent you from going fast on the bike, but the two main characters are:
Frictional Drag- from the resistance between the air flow and the objects exposed to the air flow (their surface-area).  The resistance is a function of the viscosity of the air which, although very low, is significant enough to slow you down.
Pressure Drag-  from the eddying motions that are set up in the air behind the rider as they ride.  This is also termed wake (similar to that behind a boat).  This type of drag is a strong function of the cross-sectional area of the moving body (you).

The figure below is a good way of illustrating the two.  The object on the left is mainly subject to frictional drag, that on the right, pressure drag.

There are a number of equations associated with fluid flow but probably the most important is Bernoulli’s equation:

Why is this equation so important?  It relates the drag of a cyclist to his or her velocity.  You’ve probably heard that aerodynamics become more important the faster you go and it’s all due to the V2 term – drag is proportional to the square of your velocity  i.e.  Cyclist going at 10mph    V2  = 100      Cyclist going at 20mph V2 =400  i.e. 4 times more
As you go faster, your drag increases more so.

Below is a comparative list of the various modifications that you can make to your bike setup to improve drag – however, REMEMBER: The most aerodynamically inefficient part of cyclist is the rider themselves.  About 80% of the drag of a cyclist comes from the rider – bike FIT therefore is paramount since it affects aerodynamics on the bike and in particular, the pressure drag.

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