Transfer order

Hi

can any one let me know what is the link between a sale order and the transfer order when it is created and what is the exact role of it please could you explain..

of course in simple word i will tell my requirement i have to create a TO for the delivery which throws an error saying item pick is not defined

Rather than explaining the solution it will be helpful if specify the relation and then why it happened and what should be done for solving

Thank you thanks in advance