Before there were computers, people used dice. People still use dice, the common polyhedral dice used during roleplay are d4, d6, d8, d10, d12, and d20. The number after the d represents how many sides it has. I'm not going to get into the types of solids that the dice are or whether or not a computer's function for random is just a fake random, this post isn't about that.
This about the different types of die resolution systems