(interrogative) what
(relative) which (referring to the entire preceding clause)
(relative) that, which (referring to das, alles, etwas, nichts, and neuter substantival adjectives)
(colloquial,relative) that, which (referring to neuter singular nouns, instead of standard das)
(colloquial,indefinite) something, anything (instead of standard etwas)
(dated,interrogative) why (with emphasis, astonishment or disapproval)

what; what kind of