What is A book and B book?

A Book is where the Broker doesn’t want to deal with the risk of some profitable traders group and clears all exposure made by this group earning only a commission for each trade, Thus making the Broker unaffected by this group wining but only from the volume traded. B Book is basically market making by the Broker where all of the summery exposure is left untouched counting on the traders to lose their money to the pool.