User exit and customer exit......... and BADIs

I can remember the day , i was sitting in the office in SAP-LABS around 7:30 pm and was busying in doing some development . Suddenly my phone was shouting and the voice said , rahul is it good time for interview....his one of the question was "do have knowledge about user exit or customer exit??"....and i think i replied convincingly but later i realized i need to work on it...so here is little about user and customer exit......

User Exit : We can call it FORM exit also because whatever modification we are gonna do will be in subroutine. Below is the example : 

FORM USEREXIT_XXXX....
 INCLUDE ZZUSEREXIT.......
  ENDFORM.  

User exit is basically provided to SD module so we may find for many other module user exits are not available.

Customer exit : SAP creates customer exits for specific programs, screens, and menus within standard applications. These exits do not contain any functionality. Instead, the customer exits act as hooks. You can hang your own add-on functionality onto these hooks.
Customer exits are nothing but a include in customer name space will be provided in the function module which starts with CALL CUSTOMER. You can fill them with your own source code. Technically this is an enhancement. User exits generally refer to SD module while customer exits refer to all modules like MM, SD, PP, FICO etc. Advantage:
- They do not affect standard SAP source code
- They do not affect software updates

Disadvantage:
- Customer exits are not available for all programs and screens found in the SAP System. You can only use customer exits if they already exist in the SAP System.

Types of Customer Exits
1.     Function Module exits
2.     Screen exits
3.     Menu exits

 1.     Function Module exits
Function module exits are exits developed by SAP. The exit is implemented as a call to a function module. The code for the function module is written by the developer. You are not writing the code directly in the function module, but in the include that is implemented in the function module.
           Format:  CALL CUSTOMER-FUNCTION '910'
 The naming standard of function modules for function module exits is: 
          EXIT__<3 digit suffix>

2.     Screen Exits:
Allow customer to add fields to a screen via a sub screen in an SAP program. The sub screen is called within the standard screen's flow logic.
         Format:  CALL CUSTOMER-SUBSCREEN CUSTSCR1
 3.     Menu exits:
Menu exits allow you to add your own functionality to menus. Menu exits are implemented by SAP and are reserved menu entries in the GUI interface. The developer can add his/her own text and logic for the menu.
Function codes for menu exits all start with "+".
   Format:  +CUS (additional item in GUI status) 

To know more about SAP Enhancement Frame , please click on the below URL ...please do watch..its good :)

http://www.sdn.sap.com/irj/scn/events?rid=/library/uuid/c0365021-ffd5-2b10-9085-d7de23b1aa58&overridelayout=true