Every time I see a question like this I am reminded of something V.I. Arnold wrote, which I take the liberty of quoting here:

> All mathematics is divided into three
> parts: cryptography (paid for by CIA,
> KGB and the like), hydrodynamics
> (supported by manufacturers of atomic
> submarines) and celestial mechanics
> (financed by military and by other
> institutions dealing with missiles,
> such as NASA.).
> 
> Cryptography has generated number
> theory, algebraic geometry over finite
> fields, algebra \footnote{The creator
> of modern algebra, Vi\`ete, was the
> cryptographer of King Henry~I\/V of
> France.}, combinatorics and computers.
> 
> Hydrodynamics procreated complex
> analysis, partial derivative
> equations, Lie groups and algebra
> theory, cohomology theory and 
> scientific computing. 
> 
> Celestial mechanics is the origin of
> dynamical systems, linear algebra, 
> topology, variational calculus and
> symplectic geometry.
> 
> The existence of mysterious relations
> between all these different domains is
> the most striking and delightful
> feature of mathematics  (having no
> rational explanation).