Causes:

Head trauma
senile dementia
stroke

Symptoms:

Slurred speech
Inability to speak
Stuttering
Misunderstanding
Confusion
Inability to listen

Tests/Diagnostics: cerebral angiography
CT scan
X-ray
EEG

Treatments/Cures:

None. Has been known to slowly fade away over time. A speech pathologist may be able to help in some cases.

Types of aphasia:

• Broca's aphasia (expressive aphasia)
• Wernicke's aphasia (receptive aphasia)
• Nominal aphasia (anomic aphasia)
• Global aphasia
• Conduction aphasia
• and a few weirder varieties:
  • Transcortical Motor Aphasia
  • Subcortical Motor Aphasia
  • Transcortical Sensory Aphasia
  • Subcortical Sensory Aphasia

For the record, as a medical student, I recommend Flip's definition of aphasia above as the best on this page with a little modification: "A loss of the ability to mentally produce or comprehend language." (aphasias often affect more than just speech, and may well involve reading and writing)

A*pha"si*a (#), Aph"a*sy (#), n. [NL. aphasia, Gr. , fr. not spoken; priv. + to speak: cf. F. aphasie.] Med.

Loss of the power of speech, or of the appropriate use of words, the vocal organs remaining intact, and the intelligence being preserved. It is dependent on injury or disease of the brain.

© Webster 1913.