The session  object is automatically exposed in a jsp  with the name
session. To put  and get values, it is something like:

<% session.setAttribute("name", value);
Object value = session.getAttribute("name");
%>

> what is a customizable tag  in jsp?

JSP uses the TagLibs as a way of encapsulating business logic so that it
is not directly accessible by page  designers, but is instead accessible
through certain tags. Check out the JSTL (standard tag libraries) for
some examples.

> what is difference  between page & pageconfig objects?

Not sure about this one.