"Colorless green ideas sleep furiously" is a sentence composed by Noam
Chomsky in 1957 as an example of a sentence whose grammar is correct
but whose meaning is nonsensical. It was used to show inadequacy of
the then-popular probabilistic models of grammar, and the need for
more structured models.

The full passage says:

" Colorless green ideas sleep furiously.
Furiously sleep ideas green colorless.

It is fair to assume that neither sentence (1) nor (2) (nor indeed any
part of these sentences) had ever occurred in an English discourse.
Hence, in any statistical model for grammaticalness, these sentences
will be ruled out on identical grounds as equally "remote" from
English. Yet (1), though nonsensical, is grammatical, while (2) is
not.

http://en.wikipedia.org/wiki/Colorless_green_ideas_sleep_furiously

Nonsense is an utterance or written text in what appears to be a human
language or other symbolic system, that does not in fact carry any
identifiable meaning.

Nonsense can be considered as noise.

It comes in 4 categories:

1. Semantic nonsense. the use of wrong, distorted, mis-interpreted,
self-invented or non-existing words. not making sense

2. Syntactic nonsense, words put together in a way that does not make
sense or gives non-intended meaning.

3. Contra-factual nonsense, are statements that deviate from facts.
When known by the writer or utterer to be false, also called lies.

4. Ideo-intentionalistic or self-promoting nonsense, half truths or
"as the devil reads the bible", statements that are designed to
amplify certain facts (and ignores, hides or denies other related
facts) to suit specific (normally personal) goals.

Distinguishing sense from nonsense

While Emily Dickinson wrote that:

Much madness is divinest Sense
To the discerning Eye...

The problem lies in the discernment. Distinguishing meaningful
utterances from nonsense is not a trivial task. Confronted with a
lengthy text in an unknown script, how does one determine whether
those characters in fact contained a meaningful text, or were simply
set using the equivalent of printer's pi or a lorem ipsum-style text?

The problem is important in cryptography and other intelligence
fields, where it is important to distinguish signal from noise.
Cryptanalysts have devised algorithms for this purpose, to determine
whether a given text is in fact nonsense or not. These algorithms
typically analyze the presence of repetitions and redundancy in a
text; in meaningful texts, certain frequently used words--for example,
the, is, and and in a text in the English language--will occur over and
over again. A random scattering of letters, punctuation marks, and
spaces will not exhibit these regularities. Zipf's Law attempts to
state this analysis in the language of mathematics. By contrast,
cryptographers typically seek to make their cipher texts resemble
random distributions, to avoid telltale repetitions and patterns that
may give an opening for cryptanalysis.

http://en.wikipedia.org/wiki/Nonsense