IBM's primer explaining JSTL from its origin
Part - 1: http://www.ibm.com/developerworks/java/library/j-jstl0211.html Focusses on the expressiveness of the JSTL language
Part - 2: http://www.ibm.com/developerworks/java/library/j-jstl0318/ Focusses on explaining the JSTL syntax
Part - 3: http://www.ibm.com/developerworks/java/library/j-jstl0415/ Explains the localization and internationalization topics
Part - 4: http://www.ibm.com/developerworks/java/library/j-jstl0520/ Explains accessing the SQL and XML resoruces.
Part 4 can be skipped as JSTL is not widely used for this purpose.
XMl tags can be used for parsing the XMLs.
SQL tags look similar to the concept of ASP.NET Data controls where accessing DB from presentation layer is mainly used.
All in all a very good resource for the quick grabbers and new comers to have a look at JSTL.