What exactly is the pentagram?

So my friends and i have this in side joke about the Antichrist and what not, so we draw the pentagram everywhere. We have this friend who is Catholic so we joke around with him and write in on his book sack and on his arm when he falls asleep in cl. He freaks out big time. But anyways my friend Ross rights the pentagram on my almost every day, and all my friends call me satan :) (they aren't being mean, you'd just have to know me and my friends) We've talked about supernatural stuff and the pentagram and they experiences with being in haunted places that had pentagrams every where. I know that the pentagram is satanic, but what exactly does it mean? Will writing it on my arm and body invite bad spirits? Cuz idk if it's my meds (i am bipolar) or if i am just over reacting, but it seems every time i have the pentagram written on me, i feel strange, not myself, and depressed. What is that? Is it the pentagram or me just talking crazy talk?