There are a couple of websites that explain in great detail, but in short...
JE is designed as an "insert pump" that bolts in to a fiberglassed recess in the boat, and just has a driveshaft protruding into the hull. The pump works well, but is not normally considered for High Performance applications because of the more radical water angle/path between the intake and impeller. (You lose some efficiency)
JC is nearly identical to the JG, and shares the same suction piece but has a one-piece bowl design that does not allow a droop to be bolted on. This is a good pump, and some even claim a performance increase over the JG due to a slightly different bowl design. (Longer vanes)
JG is the "standard" Berk race pump that has been used forever. It has the standard intake and suction piece common with the JC, but pressurizes into a "split bowl" that allows you to bolt contraptions to the rear of the pump such as droop snoots and different nozzles. Ride plates also like to bolt to this arrangement. When most people purchase a high performance pump, this is the design most opt for. Other companies have copied this design for it's flexibility and parts availability.
Hope this helped.