Text can be stored in different ways, the encoding of the text defines how the bytes of the file have to be interpreted by the text editor reading the file. For plain text files there is no way to store meta information about encoding, so the text editor picks its standard setting.

Under Windows most files are stored and read as ANSI, if you have a file that was stored as UTF-8, with or without byte order mark (BOM), this will mess up the formating (vice versa). Additionally MacOS, Linux and Windows all use different styles of linebreaks, but these usually only mess with paragraphs and do not add funny characters.

I'd recommend getting a text editor like notepad++ (for windows) which makes it easy to switch encoding.