Diffrence between Transaction quantity and Primary quantity?

Hello experts,

Can u please explain me exact difference between Transaction Qty and Primary Qty ???

e.g. while doing Reconcilition ,

for item,

select sum(transaction_quantity), sum(primary_quantity)  from mtl_material_transactions where inventory_item_id = 8

and organization_id in (86) and transaction_date >=  TO_DATE('31-Dec-2009 00:00:00','dd-mon-yyyy hh24:mi:ss')

and transaction_date <=  TO_DATE('30-apr-2013 23:59:59','dd-mon-yyyy hh24:mi:ss')"

i checked with above query and it gives output-->

sum of transaction qty=  -13893228

sum of primary qty =   42423

so , plz tell me what is the difference if this???

Regards,

Kunal