So who is Satan? Well, I've heard some say Satan is the "root of all evil." I've also heard people say Satan is a fallen angel who is the arch enemy of God. Let's stop and give it some thought. The only way Satan could become the arch enemy of God is if God created angles with free will. And, if God is the Almighty and Everlasting God, Creator and Sustainer of the entire universe, then doesn't He have the power to just destroy Satan just by saying Satan should cease to exist? Isn't God perfect? Well, Orthodox Judaism believes that God is perfect, and that angels do NOT have free will. (To side track, Jewish tradition believes that humans are even greater than angels, precisely because we have free will and the power to decide whether we serve God properly and earn eternal reward, or to disobey God and get punished. Angels don't have the ability to earn eternal reward, and they therefore only exist for their particular missions that God commands them to do. This is very difficult to explain, but angels are merely a representation of the Divine will). If that's the case, then who is Satan?
The answer to this question is easy to comprehend and beautiful for those who truly wish to believe it. Angels do not have free will. Angels perform certain tasks that God commands them to perform (we won't get into the details). Satan was also given a task (and he is one heck of a master at what he does). Satan's task is to test us humans with various passions, cravings, and pleasures that ultimately disconnect ourselves from God and spiritual strivings by giving in too much to this world. This world is not the only existence. After we die, we go to a much better place. But what we earn in that much better place depends on how we behaved in this world. The simple understanding of is that Satan's task is to try to make us sin. However, that is not entirely accurate. According to Orthodox Judaism, when Satan says "sin," he's really saying "DON'T sin and let me see you become great by passing my test." This is mind boggling! Everyone has assumed that Satan is evil. No. What Satan presents to us is evil; all he's doing is carrying out his mission in giving us humans the opportunities for real spiritual growth. Human beings have natural tendencies and cravings, just like animals. What really makes us great is when we transform ourselves from an animal being to a spiritual being; we transform our bodies into holy beings. It is precisely overcoming our natural tendencies to do the will of God that gains us the eternal benefits that we all desire.
One may ask, "Well, if Satan is not the cause of all evil, then how come bad things happen in the world (like illness, crime, natural disasters, terrorism, etc.). How can God, Who is perfectly good, allow such things to happen?"
The answer is not easy, but God truly is the ultimate Source of all goodness. But when bad things happen, we tend to lose our faith for a slight moment. Why? Because we can't perceive God as being bad. It must be Satan is the one who did it. No. What we perceive (with our limited human intelligence) to be bad is really part of God's Master Plan for the world. Whatever happens to a person is for his/her ultimate benefit. Although we may not see the good now, sometime later, the goodness will be revealed, either later in life through some insight into past and present events, or when we leave this world and heaven tells us the goodness. If we view the events in our lives that way, that Satan is really (quietly) cheering us on to overcome his tests, then life becomes much more beautiful. The reason why God allows bad things to happen sometimes is not about Him being angry and a God Who likes to punish. God wants everyone to earn the most pleasurable thing imaginable: basking in His Glory in the World to Come. We can't understand what that means, but the Sages say that one moment in the World to Come is more pleasurable than all the pleasures ever since the creation of the world placed into a bottle. God wants us to enjoy that, but not if we aren't first cleansed of our sins.
I hope this has given you an insight into the role Satan plays in Orthodox Judaism, and I hope my explanations can help give meaning to those in search for it.