This article is from the Martial Arts FAQ, by Matthew Weigel firstname.lastname@example.org with numerous contributions by others.
There are no absolute right answers to this question. Instead of
giving the one true answer to this, below are several different
(a) Ki doesn't exist. Everything the ki model tries to explain can be
explained with body mechanics, biophysics, and psychology. There
is no need to postulate some mysterious force. Science can
(b) Ki exists absolutely. Ki is an energy, a living force, a spirit
that can be used to increase your strength, throw people around,
etc. Subjective experience shows that ki is real. It may either
be a bio-kinetic phenomena science doesn't understand yet or the
power of the mind in union with the body.
(c) Ki may or may not "really" exist. It is a useful model. The
ki model allows you to visualize how to increase your strength,
throw people around, etc.--it doesn't matter if it exists or not.
If someone invents a better model (i.e. one that is easier to
visualize), then maybe we'll switch to it.
Of the styles that stress ki, some work on developing the flow of ki
within their bodies. An example of this approach is Taijiquan.
Other styles work on letting the ki of the universe flow through them.