Der Struwwelpeter! - The Struwwelpeter!

Der is a definite article used to indicate a specific noun or group of nouns. It is the -r on the Der (not the whole word) which marks the use of the noun (or pronoun). Sometimes the definite article is used in front of proper nouns, as in the example above. The article announces(!) the noun.

Das arme Rotkäppchen is another example, this time the -s on Da-s shows that Rotkäppchen is neuter.