一个是ing，一个是被动式。
writing是主动，written是被动，比如I am writing a book.跟A book has been written by me.从语态出发，writing是进行时，表示正在进行，written是完成时，表示写完了的。
用作形容词 (adj.)
I had already issued a written farewell to the troops.
我已向部队发表了书面的告别词。
The requirement of written form can only be waived in writing.
书面形式的要求只能通过书面的形式来予以放弃。
