It's how a new line in a text file is represented (when you press Return/Enter on the keyboard?). On Mac OS X/*nix, it is represented with a 1-byte character, while on Windows it is 2 bytes, but a *smart* text editor will display them both the same.
Here is an example file:
View attachment 66847
- If you load it in TextEdit on Mac OS X, it loads correctly. If you load it in NotePad on Windows, it won't, because NotePad doesn't read *nix-style line endings (WordPad does though). It's what makes Windows special