A jerk is a mean or rude person. They don't have to be stupid. "Bob ate all the free food. Bob is a jerk" A dud is something that fails to work. Dud is usually for objects not people. "This firework is a dud. It failed to shoot" Dud is usually used for things with gunpowder (ammo, bombs, fireworks) for other things you will likely hear "Defective" or "Broken"

jerk means a stupid person. dud means a unsuccessful person.