In bdc session method we can use three function modules.
we can pass the data into the database table thorugh the
session. whenever the session processed the data is updated
into the database table.

function modules given below.

1.Open the group by using the BDC_OPEN_GROUP
2.Insert the transaction using the BDC_INSERT
3.Close the transaction using the BDC_CLOSE_GROUP.

BDC (batch Data Communication) 
In this there are three methods
1. Direct input method
2. Call Transcation
3. Session Method

session method :
we can upload the data using three function modules 
they are
BDC_OPEN_GROUP
BDC_INSERT
BDC_CLOSE_GROUP.
If we want to call more than one transcation then we use 
more than BDC_INSERTs included between BDC_OPEN_GROUP and 
BDC_CLOSE.

there is no difference between bdc & session method. session
method comes under bdc.there are 3 types of methods to
upload data into r/3 system. those are 
1.call transaction
2.session method
3.direct input method(obselete method)

Session method is itself a type of Batch Data 
Communication.