For faster startup access to a website, it is sometimes desirable to precompile all the JSP pages on a production server, so that the first users of the website do not have to bear the performance hit of JSP compilation (first to servlet java file and then to class file)
Tomcat 5.0 and above comes with a cool ANT script that uses the Jasper JSP compiler to precompile the JSPs in a webapp. The ANT script is available here.
On running the ANT script, the "WEB-INF\classes" directory has a new package "\org\apache\jsp" that contains all the class files of the compiled JSPs. The src folder would contain the generated Java Servlet files.
If U edit a JSP page on the fly, then it would again be compiled at run-time and swapped in memory with the old one, but the classes directory would still contain the old class file.