Jinns dwell inside the earth and some of them (some of those that dwell in the earth) roam in the air. Jinns are not visible, they belong to the unseen world. However, they like to appear as reptiles, like snakes. At that time we see them.
People who don't believe in Jinns ignore something great. Jinns are very present in the world. Some of the litllest things a Jinn may do to someone are horrible nightmares. These come from unbelieving jinns (there are believeing Jinns and unbelieving Jinns). Unbelieving jinns are Satans, and satans run in the blood of human beings.
When you draw closer to God, for example reading the Qur'an out of love for the great words of God Almighty, and then you go to sleep; a jinn may attack you and make you to have a horrible nightmare. I have experienced this myself. Indeed, Satan has great enmity for seekers of knowledge.
Before sleeping, reciting the verse of the Throne (Ayatul Kursi in Arabic, V 255 of The Cow which is Surah 2) will block Satan from attacking you and make you have a nightmare. To be recited in Arabic, of course.