BAPI is faster, error validations are there, no processing 
of screens, direct database update.

BAPI is directly updates the database
BDC run through the screen flows

BAPI can't handle all flow logic checking and enhancement
put by programmer to facilitate the user requirment . 

there is advantage using bapi instead of using bdc when we
go for up gradation.

Hi,
  In additional to above answers BAPIs are more efficent 
due to this also. i.e
BAPIs are RFC enabled. so, they can use by both SAP and Non-
SAP.
Where as BDCs are not RFCs. So, they are unable to used by 
Non-SAP.

  Thank you.
  Surender.

through mails r through phone,,,,,,,,,there will be some 
personal vpn network is there for every company

that is depend on the client some clients they are using the
VPN virtual private network some clients they are using the
TELNET telecommunication network.

refresh clears the data in the internal table and free not 
only clears data but also frees memory for the same

refresh cleras the data in the internaltable body .
but free clears the data in the body and as well as in work
area.

Refresh : Refresh clears the data, at the same time it 
releases dynamically allocated memory. Predefined memory 
not released.

Free    : Free clears the data, at the same time releases 
both predefined and dynamically allocated memory. internal 
table structure will not available any more.

Refresh:
example:  refersh itab.
clear the body of  the internal table data.

free: 
example:  free itab.
. clears the data.
.predefined the internal data  and dynamically allocated 
memory.

for example : refresh itab

it will remove content of internal table...

for example : free itab

it will deallocate memory associated with internal table

screen processor

flow logic control processor.