Turning the AC current into DC current
On the other side of the transformer you find two diodes wrapped in rubber insulation. The diodes act as a rectifier, turning the AC current into DC current.
Most transformer cubes that you find around the house produce a low-voltage DC current (3 to 12 volts, and less than an amp of current). DC current is necessary because rechargeable batteries store DC current, because most electronics require low-voltage DC current and because small DC motors run directly from batteries and are the least expensive motors available.
The big question remains, however - why can't somebody standardize all of these little cubes?!