This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
The first time a JSP is accessed, it is compiled into a class file and stored in a cache. The second time it is accessed, the application server checks to see if the JSP file has changed since the class file was created. If it has not changed, the compile step is skipped and the same class file is used. If you change the JSP, the application server will detect that it was changed since the last compile and will perform the compile step again and replace the class file in the cache.