[ Thanks to Ramsai
for this link. ]
“There are many places where Python can be used in Blender. In
this article by Michel Anders, author of Blender 2.49 Scripting, we
will now look at scripts that may be used to act on certain events.
These scripts come in two flavors—script links and space
handlers. You will learn what script links and space handlers are
and how they can be used to perform activities on each frame change
in an animation. You also see how to associate additional
information with an object, how to use script links to make an
object appear or disappear by changing its layout or changing its
transparency.”