Although jinn were around pre Islam, they are definately a part of our beliefs, based in the Qur'an and Sunnah of the Proophet Muhammad (saws).
The Qur'an speaks that at creation, God created jinn from smokeless flames of fire. Jinn were created before man, but both Jinn and man were created to worship God.
Jinn can be good or evil and will be judged along with mankind on judgment day. The Qur'an is a revelation heard by both mankind and jinn, so jinn are required to follow it as well:
And We diverted a group of jinn towards you to listen to the Qur'an. When they were in earshot of it they said, 'Be quiet and listen.' When it was over they went back to their people, warning them. They said, 'Our people, we have heard a Book which was sent down after Musa, confirming what came before it, guiding to the truth and to a straight path. Our people, respond to Allah's caller and believe in Him. He will forgive you some of your wrong actions and save you from a painful punishment. Those who do not respond to Allah's caller cannot thwart Allah on earth and have no protectors apart from Him. Such people are clearly misguided.' (Surat al-Ahqaf, 29-32)
In fact, Satan is a fallen jinn in Islamic theology, not a fallen angel:
When We said to the angels, 'Prostrate yourselves to Adam,' they prostrated with the exception of Iblis. He was one of the jinn and wantonly deviated from his Lord's command. Do you take him and his offspring as protectors apart from Me when they are your enemy? How evil is the exchange the wrongdoers make! (Surat al-Kahf, 50)
What sets jinn apart from humans are the powers that they posess. Some can fly, others can change shape, some can take possession of a human's body and mind. They can be a constant stumbling block to man, or a great help.