They are spirits, just as God is spirit. Under normal circumstances they cannot be seen, but they can assume human or any other likeness whenever they wish. This is how Dad or a favourite aunt can suddenly appear to you. Demons know what they look like, what they sound like and everything about their histories, right down to their pet names for you. So, they mimic them.
Demons can speak, too.
At times, they can be heard talking through human beings. They can make people howl like rabid dogs, scream like pigs, utter vile curses and so on. In other words, demons are powerful - far more powerful than human beings. Christians should treat demons warily, but not with sanctity. They are evil, not holy.