"Do you believe in the devil?"
"If so who or what is the devil?"
The Devil is a malevolent spiritual entity described in the Abrahamic Religions but especially Christianity. In Christianity, the Devil is an archangel of God who rebelled against his Creator, and in the process incited other angels to join his rebellion. He was motivated to rebel chiefly by pride. Either he wanted to directly usurp the glory and power due to God or he thought himself as wise as God and took issue with the way that God was running things. Either directly or indirectly he appeared to the woman in the Garden of Eden and incited mankind to join his rebellion against God.
The decision by the first human couple to disobey God's command meant either that they introduced sin into the world and caused their own offspring to be born sinful and in need of a Savior OR they rebelled because man was sinful from the onset. It can be assumed that Satan desires for humans to go to Hell because either he hates them for being created in the image of God (and perhaps is jealous of such a privileged position that humans enjoy), or because he knows that doing so serves to break God's heart, or because he's headed there and wants company, or because he doesn't want humans glorifying God, or perhaps all of the above.
"What effects does this being have on our world?"
As described above, but to elaborate further: Satan according to Christianity is active in the world today, either directly or indirectly, with the purposes of swaying the behavior of the masses and the governments of the world for the purpose of stifling the spread of Christianity and eroding it wherever it exists presently. While he has already been described in the Bible as the Prince of this World, in the end of days he shall assume total control over human civilization and direct all of its resources towards the eradication of the Church, but the return of Jesus shall put an end to his reign of terror and he shall be cast into Hell.
"Would this being mind, do you think, if we call it Stan for the purposes of this discussion?"
Were such a being to exist, I suspect he would be delighted that you treat all of this as one big joke.