What is the difference between 'deaf' and 'Deaf'?

In the United States, "Deaf" with a capital "D" refers to those that use American Sign Language as their primary language. Not all deaf people are Deaf.

To be deaf is to be unable to hear. It's normally defined as being profoundly hard of hearing.

But to be Deaf is to be use sign language as your primary means of communication.

So deaf people who aren't "think hearing" but embrace being deaf and therefore sign fluently are considered Deaf