What's the actual difference between N-400 and I-485? I keep seeing both.

They're for completely different things, and I see people mix them up constantly. I-485 is the application to become a permanent resident (green card). N-400 is the application for citizenship — something you file later, after you've held a green card for the required time (usually 3 or 5 years). If you're not a green card holder yet, N-400 isn't relevant to you yet.