Shaitan (Satan) is the source of evil in the world. The plural name is Shayatin. He always tries to misguide and mislead people. The Qur'an states that Satan is not an angel but a member of the Jinn. His other name is Iblis.
When Allah created Adam, He ordered the angels as well as Iblis to prostrate for Adam. They all obeyed the order of Allah except Iblis. His argument was that Allah created Adam from clay and Iblis from the flame of fire. Accordingly, Iblis thought that he was better than Adam. Hence, Allah told him that he will dwell in Hell. Iblis asked Allah for a postponement until the hereafter. Allah granted his request. Iblis swore that he would mislead and misguide all the people except those sincere and devoted worshipers of Allah. Allah told him that only the misguided ones would follow him and that He would fill Hell with him and his followers.