I think first two sentences from wikipedia are clear enough:
A scripting language, script language
or extension language is a programming
language that allows some control of a
single or many software
application(s). Languages chosen for
scripting purposes are often much
higher-level than the language used by
the host application...
In this case the application is the browser. And about compilation:
Scripts are
often, but not always, interpreted from the source
code or "semi-compiled" to bytecode
which is interpreted, unlike the
applications they are associated with,
which are traditionally compiled to
native machine code for the system on
which they run
About 0 being equal to '', the coersion it is not necessarily achieved by a compiler; it's all about the Java script engine in runtime.
I feel sorry for taking everything from wikipedia but it's so clear and I put it quoted
PS: I find worth to paste this too:
Many people view 'scripting' languages
as inferior or somehow different than
languages that haven't achieved
popularity on the scripting scene.
Ironically, these same languages were
carefully chosen for scripting due to
their quality and versatility.