Difference between Oracle Applications 11i and R12

Database:
Database Version in 11i (11.5.9 & 11.5.10) was 9i Rel 2 where as in Release 12 its 10g R2 (10.2.0.2)

Application Tier:
Tech Stack in Application Tier consist of iAS(1.0.2.2.2) & Developer 6i (Forms & Reports 8.0.6) but in Applications R12 it is build on Fusion Middleware (10g Web Server and 10g Forms & Reports)

Sub component in Application Tier
A) HTTP Server or Web Server in R12 is Version 10.1.3 which is built on Apache version 1.3.34. In apps 11i it is Version 1.0.2.2.2 built on Apache Version 1.3.19
B) Jserv in apps 11i is replaced by OC4J (mod_jserv is replaced by mod_oc4j)
C) Forms Version 6i in Apps 11i is replaced by Forms 10.1.2.0.2 in R12
D) Reports Version 6i in Apps 11i is replaced by Reports 10.1.2.0.2 in R12
E) JDBC version is changed from version 9 in apps 11i to version 10.2.0 in Apps R12
F) modplsql or mod_pls is removed from Apps R12 (What will happen to my mod plsql applications- coming soon* )
G) Java processes use JDK/JRE version 1.5.0 in R12 against JDK version 1.3.1 or 1.4.2 in Apps 11i
H) For various environment variable changes check below picture

I) New top INSTANCE_TOP added in Release 12 for configuration and log files

From: http://www.dba-oracle.com/t_applications_differences_11i_12.htm