Ray Tracing, in a one-line description, is a method that allows you to
create stunning photo-realistic images on a computer.  All you need is
a computer, some ray tracing software, a little imagination and some
patience.

The first stage of creating this masterpiece is to "describe" what it
is that you want to depict in your picture.  You may do this using an
interactive modelling system, like a CAD package, or by creating a text
file that has a programming language-like syntax to describe the
elements.  Either way, you will be specifying what objects are in your
imaginary world, what shape they are, where they are, what colour and
texture they have and where the light sources are to illuminate them.
Having done all of this, you feed it into your ray tracer, sit back and
wait.

And wait...

That's the main drawback of ray tracing - it's not fast.  The software
actually mathematically models the light rays as they bounce around
this virtual world, reflecting, refracting and generally having a good
time until they end up in the lense of your imaginary camera.  This can
quite literally involve thousands and millions of floating-point
calculations and this takes time.  Tracing images can take anything
from a few seconds to many days.  It's a long process, I know, but the
results can make it all worth while.

Ray tracing isn't the only method for creating photo-realistic
pictures.  There are packages like 3D Studio which uses scanline
rendering, Radiance, which uses radiosity, and so on.  Although these
don't count as ray tracing, the methods you use from one system to the
next are often sufficiently similar to warrant their discussion in this
group.  So if you think it's relevant, feel free to bring it up.  These
systems will be mentioned in a little more detail later on.