The Hebrew word "Satan" means "Hinderer." To hinder someone means to hold him back, to try to prevent him from doing something. G-d created the Hinderer to give us work to do in this world. Satan is here to make things difficult for us, so we can overcome our evil temptations, and PASS the test. That is the purpose of Satan. Satan is an angel whose purpose ("job") has been determined by G-d.
Temptation is there to try and deter us. It gives us the ability to do the wrong thing. More importantly, it gives us the ability to look at evil and refuse to do it. By presenting us with the opportunity to do evil, it gives us the ability to choose between good and evil. The ability to choose between good and evil is what gives us free will.
So, in order for us to work for the good that Hashem wants to give us, the good of the World to Come, we need something to deter us. That is the ability to do evil. Satan is our Evil Inclination (Yetzer Hara). The Evil Inclination tries to prevent us from doing good, because Hashem (G-d) has commanded the Evil Inclination to do that. Why? To give us free will.
Each of us every day fights with Satan. We all have temptations, throughout the day. But we, as the Children of Israel, have the power to overcome even angels, if we work at it. Therefore, the **Talmud says that men are greater than angels, for we can fight with an angel (Satan) and win.
The truth is that Satan has a job to do, just like every other angel. And angels have no free will. They do as Hashem (G-d) commands them.
So now we have to revise our understanding of Satan. Satan is not a fallen angel. Satan is merely an angel with a dirty job. Satan does not have a rival kingdom. Satan is not in competition with G-d, and Satan does not want followers or worshipers. He's not even happy when people obey him and sin.
