ASCII mode is used to upload text type files such as HTML, PHP, Perl and other files that were created in a text editor.

Binary mode is used to upload non-text type files such as images, sound and video files.

Some scripts will specifically say that you must upload a file or set of files in a specific mode. You can set your FTP program for ASCII or Binary mode prior to uploading the files. Be sure to set your FTP program back to auto when done. Normally auto mode will determine the type of file and change the setting automatically.