In this capitalized form, the word "Deaf" generally implies that the person doesn't hear well enough to understand speech from their hearing, alone, but it also means that they consider themselves part of the "Deaf Community".
People in the "Deaf Community" typically use sign language. In the US, they use American Sign Language (ASL), but in other countries, just like spoken languages, sign language varies greatly.
Not everyone who is deaf, considers themselves "Deaf". It's a personal decision.
Most, but not all, people who consider themselves "Deaf" were "born deaf". That means they were born without sufficient hearing to acquire an auditory language.
About 90% of all children who were "born deaf" are born to hearing parents.
Many Deaf people actually have some hearing. Many wear hearing aids so they can hear "environmental sounds" but do not have sufficient hearing to be able to understand speech from the audio, alone, though they may speechread quite well.
Most Deaf people have accepted their deafness and do not consider it a "disability" ... indeed, it is not a disability when they are among others who converse quite fluently in ASL.