Most of the time when you're counting things, you can get away with only using 십, 백, 천, and 만. For really large numbers, 억 is your friend (100,000,000). But beyond that, you'll rarely (if ever) find yourself using. However, if you start reading articles about economics, or learn biology, or physics, or mathematics and other fields where you'll need very large numbers, these won't be enough. So let's talk about everything above 억.