WTF is water injection?

Title says it all.

From: http://www.aquamist.co.uk/

The main function of these systems
is to suppress detonation caused by
high temperature and pressure developed
within the combustion chamber when
the effective compression ratio has
been taken beyond the auto-ignition point
by either a turbo or a supercharger.
Water, with its high latent heat content,
is extremely effective for controlling
not only the onset of detonation but also the
production of oxides of nitrogen in
the modern leanburn engines.

Essentially, the ability to lower the temp. of the intake charge. Of course this is purely supplemental and should be used in conjunction to an intercooler.