The purpose of the forum is to discuss topics related to the lessons. Your question is falling more into the technical domain while the program is mainly for functional topics. However I will briefly touch on this topic here although it requires a detailed white paper:

If you are asking from forms development point of view then the both types of forms are there in R12:
Oracle Forms (Think Java applet based pages)
JDeveloper (HTML like pages)
Almost 80% of the pages are written in Forms. Some new setup screens and self service pages are written in JDeveloper.
Irrespective of what the media is saying about the future direction, in my opinion the practical reality is that forms pages are best for the date entry purpose. However there are some advantages to the Jdeveloper pages as they are fast to load the first time around. That is why they are used mainly for self service purposes: where customers, suppliers, employees connects to the apps.
Companies are still developing new custom forms using Forms 10g.
The main portion of the middle tier seems to be Java based. The back end is split between PL/SQL and Java. The main chuck of the code still resides in PL/SQL packages.