JSP or Java for web applications. Java is perhaps the most popular language for enterprise level applications, so professional programmers can take their skills to the web. JSP means Java Server Pages and consists of JSP tags mixed in with html. It is best used as the presentation layer of a Java web application, although you can build JSP-only applications.
The logic behind the application is normally hidden below a JSP and is a pure Java program called a servlet. JSP's are also compiled into servlets by the JSP engine the first time it is executed for faster execution time on later requests.
Java is a big and complicated language so it can be hard to learn. One Java benefit is that it enforces good programming techniques, so if you learn Java you will probably become a decent programmer too.