Is it my machine doing it while it's downloading? Or is it already in the file. What is a corruption? I looked in the sticky posts, but couldn't find an answer.

One cause of a corruption is that the file is already corrupted at its source. And there's many of them out there! I think another reason is if there's any momentary interuptions in connection or data clashing along its path to your computer may result in incomplete info or corrupted data. Luckily LW has a method of being able to detect corruptions in a dwnlding file & attempts to redwnld those few nibbles/bytes/bits/kbits to replace/fill-in those parts. It's most likely not your computer at fault. (But get a mac computer instead just to be on the safe side! - Joke!)