The main difference between Italian sausage and hot Italian sausage is the amount of heat or spiciness they contain.  Traditional Italian sausage contains herbs and spices such as fennel, oregano, black pepper, anise and red pepper flakes that give it a mild flavor.  On the other hand, hot Italian sausage contains more chili powder or crushed red peppers to increase its level of spiciness.  Generally speaking, hot Italian sausage is much spicier than regular Italian sausage.