Many people do not realise that the word "deaf" covers a vast spectrum of hearing loss.
It may range from complete (or profound, to use the audiological term) deafness, where nothing at all can be heard, to a very slight hearing loss where one may experience difficulty in catching everything that is being said. Between these two extremes, of course, is a vast range of hearing loss. Very briefly, the range might go like this: profound/severe/moderate. I am severely deaf, which means that I can hear pretty much everything you throw at me with my hearing aids in - right now I can hear the noise of my fingers clattering away on the keyboard, and the faint hum of computers in the background - but once you take out my hearing aids, I can hear nothing except for very low sounds. (This means I can hear you if you're male and you talk right into my ear.)
Of course, this only goes for me, and someone else who describes themselves as severely deaf may hear differently again. I have a friend who is completely deaf in one ear - she has no useful hearing in it at all, and gains no benefit from a hearing aid in that ear - and partially deaf in the other, in which she does have a hearing aid. Like me, she communicates orally, through speech. She is slightly deafer than I am, however, and the telephone for her is not so useful as it is for me. I can manage quite well on the telephone, if you're prepared to spend some time making sure I've got what you said; she can't. She is also much more reliant on lipreading than I.