Put simply, extractors are pipes leading directly out of the
exhaust ports which allow exhaust gases to flow more freely out. As
the exhaust gases pass through pipes, they tend to compress at
certain points, causing high-pressure spots that restrict the gas
flow. Extractors are shaped, or tuned, in such a way that the
high-pressure spots do not occur.
Backpressure refers to the pressure created by residual exhaust in
the combustion chambers (which are never entirely expelled in the
exhaust stroke). Engines are designed with backpressure in mind, so
that the compression is optimum when the residual exhaust gases and
the fuel-air mixure (taken in during the following intake stroke)
are compressed.
If you add extractors to the car, there is less residual gas left
in the chamber than was accounted for in the engine's original
design. In turn, the pressure of in the chamber at the top of the
compression stroke is lower than it would be in the original
specifications, resulting in less pressure when the fuel-air
misture is ignited. As a result, the engine loses low-end
torque.
If you add extractors to the car, you need to be careful to ensure
that the intake is also modified such that the pressure at the top
of the compression stroke is the same as specified in the original
design (i.e. it makes up for the loss of backpressure). That way,
you don't lose low-end torque, and because the exhaust gases flow
out more freely, you get better power at high revs too, ecause the
exhaust stroke saps less power.
Well, that's my understanding of it anyway. 
