USER HOOKS API in Oracle apps

 What  is  User Hook ?
Where  it  is  used?     
Why we need it ?       
How to implement in oracle apps?

OUT LINE  OF  USER HOOKS  :----
--------------------------------------------------
Creating  User  Hook  :  Oracle HRMS
Step 1: Find the API for which HOOK has to write.
     There are mainly two tables:
                                                             1)HR_API_HOOKS
                                                             2)HR_API_MODULES
Note:   For understanding types of hook search in oracle metalink.
Step 2:   Create   A    PL/SQL    Procedure   which   fits    requirements.
Step 3:   Now  Register  created  procedure  into  Required Hook:
             API for Registering User Hook:
                                  . HR_API_HOOK_CALL_API.CREATE_API_HOOK_CALL
                                  . HR_API_HOOK_CALL_API.UPDATE_API_HOOK_CALL
                                  . HR_API_HOOK_CALL_API.DELETE_API_HOOK_CALL
Step 4:     Once Hook registered. We  need  to  run  the  Pre Processors  which  will  taken care by DBA.

Step 5:   Once done  with  all abov e steps verify  the  status  of  user hook  in t he table called:
                              HR_API_HOOK_CA