JSTL is developed under the Java Community Process, in the JSR-052
expert group. The purpose of JSTL is to work towards a common and
standard set of custom tags.
It
encapsulates simple tags as the common core functionality to many Web
applications. It also provides a framework for integrating existing custom
tags with JSTL tags.
A tag library is a set of actions
that are used within JSP pages. It
encapsulates a wide variety of functionality that can be broken down into
specific functional areas. JSTL is a single taglib, which is exposed through
multiple Tag Library Descriptors (TLDs) where each TLD can have its own
namespace, or prefix.
Read more at
http:/www.roseindia.net/jstl/introduction.shtml