BAPI STRUCTRE

HI,

I am samba, new for bapi consept; please help me on bapi. what is line structure. why we are not use line structure in bapi, and why bapi does not taken the same structures used in existing applications.

And explain about key field and event in bapi, what we will do on events and where we will write the code for this event, what happen by this event and key field.

Please explain clearly..

Thanks

Samba