My first impulse was to say "they are question words".
If you are interested in finding out what part of speech they are, it would be a good idea to consult a dictionary. They give reference to word classes at the very beginning of an entry. 'How', for instance, belongs to two classes.
You've asked a remarkably similar question before. The answer is still the same: consult a dictionary; it will tell you what part of the speech a word is.