Speech is preserved but language content is incorrect. This may vary from the insertion of a few incorrect or nonexistent words to a profuse outpouring of jargon. Rate, intonation and stress are normal. Substitutions of one word for another ("telephone" --> "television") are common. Comprehension and repetition are poor.
e.g. "I called my mother on the television and did not understand the door. It was too breakfast, but they came from far to near. My mother is not too old for me to be young."
If excessive, this may be confused with the psychiatric signs of "pressure of speech" and "word salad".
Patients who recover from Wernicke's aphasia report that whilst aphasic they found the speech of others to be unintelligible and ... even though they knew they were speaking they could neither stop themselves nor understand what they had just said.
See how this is different from expressive aphasia / Broca's aphasia.