JSTL, which stands for JavaServer Pages Standard Tag Library, is a collection of custom tags that provide a standard way to access JavaBeans components and simplify JSP coding. It was developed by Sun Microsystems, and is now maintained as part of the Apache Jakarta project.

JSTL consists of a number of tag libraries that can be used to perform various tasks such as conditional processing, looping, formatting, and database access. The tag libraries are implemented as JAR files, and can be included in a web application by adding them to the application's classpath.

JSTL provides several benefits over traditional JSP scripting. It separates presentation logic from business logic, making code more maintainable and easier to read. It also provides a more concise syntax for common tasks, which reduces the amount of code that needs to be written.

JSTL is widely used in Java web development, and is supported by most web application servers and IDEs. Some of the common JSTL tags are:

1. <c:if> - Evaluates a condition and includes the body of the tag only if the condition is true.
2. <c:forEach> - Loops over a collection and executes the body of the tag for each item in the collection.
3. <c:set> - Sets a value in a variable.
4. <c:choose> - Provides a way to test multiple conditions and execute different blocks of code based on the conditions.
5. fmt:formatDate - Formats a date according to a specified pattern.
6. sql:setDataSource - Sets up a data source for a SQL query.

To use JSTL in a JSP page, you need to include the JSTL tag library in the page. This is typically done by adding the following line at the top of the JSP page:

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

This line includes the JSTL core tag library, which provides basic functionality such as conditional processing and looping. Other JSTL tag libraries can be included in a similar manner.

Once the tag library is included, you can use the JSTL tags in the JSP page. For example, the following code uses the <c:if> tag to include a block of code only if the condition is true:

<c:if test="${someCondition}">
  <p>This will be displayed if someCondition is true.</p>
</c:if>

JSTL provides a powerful and flexible way to simplify JSP coding and improve code quality. By using JSTL, you can create more maintainable and efficient web applications.