Written sources are all the objects that give us written information.
The Rosetta Stone (see picture) is a good example of a written source.
Other examples are: Anne Frank's diary, the Bible, a newspaper from World War 2, hieroglyphics on a temple wall, books, documents, tax records or adverts.
As long as the text gives us information about the time we are studying it is a written source.
Non-written sources are objects that give us non-written information.
Ötzi's body is a good example. Other examples are:
a Greek vase, a Medieval castle, a Viking ship, a uniform from World War 1, paintings, photographs, even films.