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 (http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?productId=100140850&storeId=10051&langId=-1&catalogId=10053&ci_sku=100140850&ci_kw=%7bkeyword%7d&kwd=%7bkeyword%7d&cm_mmc=shopping%2d%5f%2dgoogleads%2d%5f%2dpla%2d%5 f%2d100140850&ci_gpa=pla)

An examole of a 2 pole breaker:
http://www.homedepot.com/buy/electrical/breakers-distribution-load-centers/qo/30-amp-double-pole-circuit-breaker-61132.html

Then a split, double or tandem breaker, example:
http://www.ebay.com/itm/NEW-THOMAS-BETTS-TBBD2020-TWIN-20-AMP-PLUG-IN-BREAKERS-FREE-SHIPPING-USA-/180783190184?pt=BI_Circuit_Breakers_Transformers&hash=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.