This figure shows a conventional cell with cannonballs at each point of the Bravias lattice.
Note that there are four balls in each conventional cell. The primitive cell, however, holds only one ball in each cell.
Putting the conventional cells together, you get a nice little crystal.
This is a slice trough the (111) plane. It can be seen that this is the best way to pack the cannonballs. Actually, the fcc Bravias lattice has the same configuration as a certain hexagonal crystal structe with a three atomic bases (ABCABC...). Note that the hcp (hexagonal closed packing) crystal structure is a bit different. It has a two atomic bases (ABAB...).