The honest answer is this: Salami (sliced salami), pepperoni (sliced spicy salami), and sausage (pre-cooked fennel-flavored uncased/crumbled pork sausage).

In the US, “sausage” tends to generically refer to uncured, fresh, or raw sausages, often really meaning “ground meat mixed with herbs and spices sometimes in a tube or casing (but not always).”