According to what I understand they are mentioned in the Qur'an as beings created by Allah along with angels & humans. Like humans they are said to have free will, can be good or bad, are beings created from smokeless fire, etc... Trying to give you as much information as I can I performed a quick search and came across something that indicates the Qur'an refers Satan as a djinn who was cast out of Paradise for refusing to bow down to Adam when angels & djinn were told to do so, which he could do having free will.
Like other spiritual entities, such as demons, Djinn are said to be able to posses humans, travel great distances, etc.. but do not have the power or capabilities to grant wishes as Disney portrays them nor are they bound to do so. So people attempt to bind djinn and you will likely see advertisements online where people "claim" to sell items with djinn bound to them. There are also different categories of Djinn the only category coming to mind at the moment being Ifrit as I've seen it repeatedly as it often appears in fictional sources. There are many online resources about the Djinn which can be used to further your knowledge. I'm also certain you can find more information on this very site.