Korean essence vs serum vs ampoule - what is the difference?

I am trying to understand Korean skincare products but I am confused by all the different categories. What is the difference between an essence, a serum, and an ampoule? Do I need all three in my routine or is that excessive? In what order should these be applied? I have seen products like COSRX Snail Essence, various vitamin C serums, and ampoules that all seem to target similar concerns. Can someone explain when to use each type and recommend some good Korean products in each category? My main concerns are hydration and brightening.