http://www.ietf.org/rfc/rfc0959.txt,
Chapter
3.1.1. DATA TYPES
As you may know, End of lines are encoded differently between Macintosh, Linux, MsDos. When you open in windows a file created in linux, with notepad for example, you will realize that there is no end of lines (there is no carriage return)
When you use
ascii transfer type, the ftp protocol will make the change for you.
Example, if you transfer from Linux-> msdos it will change all end of line to conform to msdos. Opening it in notepad will show you that (msdos') end of lines are there.
Take care, if you transfer a jpg in ascii format and the ftp protocol recognizes end of lines: byte 0A (while in fact it's not an end of line but a pixel), you will end up with 0D0A (if the destination is windows). Then your image won't be viewable anymore. For this you need to use the type
bin (IMAGE in ftp specification)