Binary mode is intended for transfers of files that are not clear text. This includes pictures, audio files and other files like MS Word .doc files. During binary FTP transfer the files are transferred as-is, without any changes at all. However, when using ASCII transfers - the other choice - the client (or server, depending who is sending the file) will compress the file. This compression breaks binary files, but is very efficient on text-only files like source code and well, just plain text. In short, ASCII transfers are faster for text files but break binary files.