Der, die, das — the definite articles

Der, die, das are the definite articles of German — all three mean the, but each belongs to one grammatical gender: der masculine, die feminine, das neuter. The article is not decoration; it is the hook every case ending hangs on, which is why the singular most important vocabulary rule is: never learn a noun without its article.

Gender is mostly arbitrary (das Mädchen — the girl — is neuter), but not entirely: suffixes like -ung, -heit, -keit predict die, -chen predicts das, agent -er predicts der. The complete pattern table lives here; the full case tables are here.