JSTL (JSP Standard Tag Libraries) is a collection of JSP custom tags developed by Java Community Process, www.jcp.org. The reference implementation is developed by the Jakarta project, jakarta.apache.org.
The latest version of JSTL is JSTL 1.1, which requires a JSP container that supports the Java Servlet 2.4 and JavaServer Pages 2.0 specifications. Jakarta Tomcat 5 supports these specifications.
The previous version is JSTL 1.0, which requires a JSP container that supports the Java Servlet 2.3 and JavaServer Pages 1.2 specifications. Jakarta Tomcat 4 supports these specifications.
Since I have Tomcat 4.1.18 installed on my machine, I will talk about JSTL 1.0 only in this section.
The goal of JSTL, as described in the specification, is to help simplify JavaServer Pages page authors' lives. To achieve this goal, JSTL has provided custom tags for many common JSP page authoring tasks that require scripting statements to manipulate server side dynamic data.
JSTL offers tags through 4 libraries:

• core - Basic scripting functions
• xml - XML processing
• fmt - Internationalization of formatting
• sql - Data base accessing

Click here for JSTL Specification