(1) Arbitrary Gibberish: Examples might be 'sgjkdf' and 'wiouein' or 'dabalobidra'. These nonsense words have no meaning, no association, and are just an artifact of our ability to run through combinations that our language does not in fact use.
(2) Mock Vocabulary: Mock vocabulary has no sense in itself, but exists in a context that makes it work as if it does. Mock vocabulary is distinguished by Alice's Test: "Somehow it seems to fill my head with ideas -- only I don't exactly know what they are!" An obvious example is 'vorpal' from Lewis Carroll's "Jabberwocky":
One, two! One, two! And through and through
The vorpal blade went snicker-snack!
He left it dead, and with its head
He went galumphing back.
Alice's Test distinguishes mock vocabulary from arbitrary gibberish, which doesn't "fill my head with ideas". It also distinguishes mock vocabulary from the next kind of nonsense word, which does fill the head with ideas, but which depends on your knowing exactly what they are.
(3) Incompatible Combination: Examples of nonsense associated with definite ideas would be 'goat-stag', 'round square', and the like. This kind of nonsense has definite sense, arising from the component parts; it's just that those parts should not go together. You know what a goat is; you know what a stag is; you can't have something that is both simultaneously.
(4) Representative Gibberish: Gibberish is not completely divorced from language. Sometimes you want to say that something is gibberish, and one way you can do that is by labeling it with gibberish. 'Blah blah blah' is possibly of this sort; 'blictri' in logic examples is certainly of this sort. It's supposed to be gibberish; but because it is representative gibberish, it functions as a real word standing in for other kinds of gibberish, or (sometimes) for things that are not gibberish but practically speaking might as well be gibberish -- which means that even though it is nonsense it practically has a meaning. Everybody knows what 'blah blah blah' means in the Gershwin lyrics:
Blah blah blah blah blah blah your hair
Blah blah blah your eyes
Blah blah blah blah care
Blah blah blah blah skies
The whole point is that 'blah blah blah' is gibberish; it conveys that while lover's songs (especially those in movies) may put meaningful words in the place of 'blah blah blah', none of that meaning really makes much of a difference; and because it does convey this, in a paradoxical way it has meaning. That is what representative gibberish is: nonsense words that remain nonsense, but because they stand in for other things, act in language as if they had senses.
