JSTL (JSP Standard Tag Library) is a collection of useful tags for controlling flow, iteration, and other common tasks in JSP pages. It includes tags for core functionality like iteration and conditionals, formatting numbers and dates, SQL queries, XML processing, and common functions. The tags are organized into libraries for core, formatting, SQL, XML, and functions. Using JSTL tags removes scriptlet code from JSPs and makes the code more readable and maintainable.