It is a front end tool which consist of forms and reports and use plsql as a coding language
We have done data conversion for1.Suppliers2.AP Invoices3.Customers4.AR Invoices
As we basically use 11.5.9 version hence its better to tell that only.
If you are technically strong in Oracle D2k then it will take very less or near abt one month to learn abt Oracle apps.
The best example for user exit which i used is FND FORMAT_CURRENCY.