Pre-exit, Post-exit and over write exit

Pre-exit --> This is a method which triggers before the original method when you enhance the SAP standard component (at run time).
Post-exit ---> This is a method which triggers after the original method when you enhance the SAP standard component(at run time).
Overwrite-exit---> This is a method which over writed the original method when you enhance the SAP standard component at run time.