psql & java

I want to write a Java applet that retrieves/updates data in a psql
database.  I am running Apache and psql on a single OpenBSD server.  I
currently use PHP for dynamic pages; which has been fine up to now for
presenting data, but I can see the day coming where some processing at the
client side would be useful; so I want to take a closer look at java.  I
have been told by someone who knows little more than I do, that I need
servlets to get at my data with java applets.  When I go to the apache
site, I see references to Jakarta, Tomcat, Java-Apache, JSERV and JSSI.
But I couldn't find summaries that tell what these things are.  Can someone
tell me exactly which of the above components (and more?) I need to install
to make this work?  How do they fit together?  Do I also need JDBC?  Does
JDBC require ODBC?

Frank