What is th difference between a duplex 20/20 breaker and a 20/20 twin breaker siemens brand

Here is an examole of a quad breaker:
One 50 Amp 2 Pole and Two 20Amp 1 Pole Breakers BR Quad Circuit Breaker-BQ2502120 at The Home Depot

An examole of a 2 pole breaker:
http://www.homedepot.com/buy/electri...ker-61132.html

Then a split, double or tandem breaker, example:
http://www.ebay.com/itm/NEW-THOMAS-B...item2a178490a8

With the quad you can get, 2 240 volt citcuits, 4 multicircuits, or a combination.
The 2 pole breaker is 1 240 volt circuit.
The Split/double allows 2 120 volt circuits on the same phase.