What's the plug pair called?
I don't think there's an "official" name for it, but it's usually referred to as the "ground power connection" or something similar. Also, there's usually only one connection; only the biggest commercial aircraft need two connections simply because one connection can't supply enough power.
What's the ISO/whatever standard involved?
What do they look like close-up?
I wasn't able to find a copyright-free picture, but here's a PDF catalog from an aircraft parts supplier that has some pictures. (Note that this catalog also has a bunch of rarer plugs in addition to the common ones.)
Is it pretty standard today worldwide?
The international nature of aviation kind of militates towards better standardization than most other fields. It's not perfect, but you're a lot more likely to find a compatible plug for your plane in a foreign country than you are for, say, your electric razor.
Is the plug identical for the hugest airliners as for say commuters?
Is the power specs identical for the hugest airliners as for say commuters?
The plug is based on what kind of power the airplane needs, rather than the size. So, all airplanes that take 120V 400Hz power will (usually, see above about standardization) have the same plug.
Is the plugpair used on any non-aviation (say, industrial) cases?
No, not that I know of.
Do some craft have more than one type of plughole (ie, to allow for different airport standards?)
It's a pretty big world, so I'm sure that some airplane, somewhere, has multiple plugs.
And military aircraft?
That I don't know. Someone else will have to answer that one.
Do any small personal craft (I mean 2-4 seater craft) use the same plugpair, or in some cases do they just use a domestic-like plug (rather as a motorhome may have), or generally do small craft never have this?
Generally, most small aircraft use 28V DC power, the connector for which can be found in the catalog I linked to.
Is there a switch (for example, perhaps near the pylon in the first photo?), or a switch near the plug, or does it just hot connect?
Generally, the switch is in the cockpit rather than next to the plug, and it's the pilot's job to make sure there's no hot connect/disconnect.