The blog All Things Metathesis has a nice entry about a large scale metathesis reaction, and how the changing economy has impacted it.
Metathesis can be (over)simply stated as moving double bonds around. The reaction is actually a combination of breaking and reforming the double bonds, but from looking strictly at the inputs and outputs, it looks like the double bonds are moving around. This is obviously important to the polymer industry as the double bonds can be used in polymerization schemes.
What I never knew was that metathesis is an equilibrium process, so that it can be run backwards - an important part of the post. Read the entry and see why.