Originally quoted by Ola Daniel.
This is from the jsp specification - Do read the warning carefully.
The value is a fully qualified Java programming language
class name, that names the superclass of the class to which
this JSP page is transformed (see Chapter JSP.11).
This attribute should not be used without careful
consideration as it restricts the ability of the JSP container to
provide specialized superclasses that may improve on the
quality of rendered service.