Damage. To broca's area of the brain.
Aphasia. Aphasia is a disorder of language processing - it may take the form of poor comprehension (receptive aphasia) or inability to think of words (expressive aphasia). The latter is more common. The individual is frustrated as they know what they want to say but can't find the words. Sometimes they can use a keyboard or point to pictures to communicate but not always. Speech therapy and time can help.
Inability to speak. Aphasia after stroke occurs after the portion of the brain that controls language is injured from insufficient blood flow (often a clot). Afterwards, patients appear attentive, and at times have the appearance of wanting to communicate, but are unable to either understand verbal or written language, or utter a sentence, or some combination of the two. Prognosis often depends on initial severity.