I'd understand greebles to be simply the "random" crap that you add to models to get things interesting. This wouldn't be like "wrinkles" on a face because that is necessary for a model, but for example the sci-fi stuff, but it could be things like pipes, cogs, resistors, that kind of thing, that while are needed for interest, aren't necessary to directly show that a ship is a ship, whereas you "need" wrinkles on a face to show age, hence the difference. I guess it could also apply to organic models, but from what I've seen, the term is mostly used in sci-fi art.
As far as the creation, I think it could be either as modeled geometry(either sitting on to of, or part of, or displaced from the original model), or bump/normal mapped onto it. The method actually used would likely depend on the where the final product is ending up. Also, modelling it could be used to create/bake a normal map for it, but in any case, I think it ALL counts as "greebles".