15 amp vs 20 amp difference?

Been wondering about this for a while, some power conditioners (Shunyata in particular) and other equipment spec. to use a 20 amp power cord. What is the physical difference between the two? Would the plugs for iec and male input be different and not fit into the regular outlets (ie like the Oyaide GXS or PS Audio Ultimate or do they require a different type specifically just for 20 amp usage? I do realize the amp rating (that they'd draw) between the two would be different. Your help would help to further edudate me, Thanks.
The blades of a standard 20A IEC receptacle are horizontal(http://www.parts-express.com/pe/showdetl.cfm?Partnumber=110-429), those of a 15A are vertical(http://www.parts-express.com/pe/showdetl.cfm?Partnumber=110-432). The IEC plugs will not interchange. If you are asking about wall outlets and AC plugs; the basic configuration is also different for 20A and 15A(125V), a true 20A plug will have one horizontal blade(Nema 5-20), and a 20A outlet a dual-use(15 or 20A) slot.(http://www.elect-spec.com/nema_plgsokt.htm)
Rodman99999, Thank you! I have never known (or investigated the differences before and your inclusion of the websight and the pictures that were provided helped to make all perfectly clear now for me. Thank you again for your help. :)
No problem. =8^)
Seems to be common on the Shunyata products to use a 20amp IEC and the use a 15A AC plug with just the vertical plugs on the other end.

Just make sure that the cable you want to use is rated for 20A, not that you would usually ever draw that much current.
I've never actually seen a 20A audio power cord with a true 20A plug. I just recently bought a 20A ZU Mother, and it had a 15A Wattgate plug.
Shunyata Research equipment is UL/CSA Listed. Beings they use 20 amp receptacles they have to use a 20 amp IEC male connector to get the UL seal of approval.

For some reason a power cord with a 20 amp IEC connector and a 15 amp plug is ok with UL..... That one doesn't make any since to me. By allowing the configuration it is not idiot proof. Nothing prevents a user from plugging loads designed for a 20 amp branch circuit into a 15 amp branch circuit.....
The issue lies in the wiring in the walls. The NEC specifies that the temperature of a wire cannot exceed a certain threshold when the maximum current is continuously used. This is specified to prevent electrical fires occuring inside of your walls from an overheated wire. So the breaker is then matched to the wire as are the outlets (here a 15A circuit). For this reason, changing the outlet to a 20A is risky without also changing the wiring from the breaker box to that outlet.
For this reason, changing the outlet to a 20A is risky without also changing the wiring from the breaker box to that outlet.
And that is why NEC says a 20 amp receptacle shall not be installed on a 15 amp branch circuit.....

Hah! I just received a Running Springs Mongoose 20A cord, and noticed it has the exact same pairing as the Shunyata and Zu referenced above... a 20A IEC connector, but a 15A rated standard plug! I thought it might have been a production error, but I guess not. Interesting!