There is often confusion over the terms "hearing impaired," "hard of hearing," "deaf," and "deafened," both in definition and appropriateness of use.
The term "hearing impaired" is often used to describe people with any degree of hearing loss, from mild to profound, including those who are deaf and those who are hard of hearing. Many individuals who are deaf or hard of hearing prefer the terms "deaf" and "hard of hearing," because they consider them to be more positive than the term "hearing impaired," which implies a deficit or that something is wrong that makes a person less than whole.
"Deaf" usually refers to a hearing loss so severe that there is very little or no functional hearing. "Hard of hearing" refers to a hearing loss where there may be enough residual hearing that an auditory device, such as a hearing aid or FM system, provides adquate assistance to process speech.
"Deafened" usually refers to a person who becomes deaf as an adult and, therefore, faces different challenges than those of a person who became deaf at birth or as a child.
Deaf, deafened, and hard of hearing individuals may choose to use hearing aids, cochlear implants, and/or other assistive listening devices to boost available hearing. Alternatively, or in addition, they may read lips, use sign language, sign language interpreters, and/or captioning.
People who are deaf or hard of hearing may have speech that is difficult to understand due to the inability to hear their own voice.
For more information on appropriate terminology, consult the resource at the National Association of the Deaf titled What is Wrong with the Use of these Terms: "Deaf-mute", "Deaf and dumb", or "Hearing-impaired"? 
-  What is Wrong with the Use of these Terms: "Deaf-mute", "Deaf and dumb", or "Hearing-impaired"?
-  National Association of the Deaf
-  Hearing Loss Association of America