Correct, but note that for "door knob" (two separate words) dict.cc includes a suggestion to see also [siehe auch] "doorknob" (no space), where one will see that the more common term is "Türgriff", and that there is no mention at all of "Türknopf". In fact, dict.cc has no entry at all for "Türknopf", so this strongly hints that it is not much used at all.
Actually, it's funny that the "k" in words like "knob" or "knot" is no longer pronounced in English. :)
In the Middle Ages, "kn" at the start of a word was still pronounced as "kn" and not as "n" in English. That's the reason why these words are spelled with "kn", and not just with "n". You can listen to the medieval pronunciation of the word "knight" here ("Go to: Knight", then click on the microphone symbol on the right). In medieval times, the word "knight" was not only pronounced with the "k", but also with the "gh" (= German: ch).
German has something English doesn't have, which is grammatical gender. This means that every noun is assigned a "gender". There are three in German, and they are masculine, feminine, and neuter. Of course, this is simply a grammatical function and not a way of saying an object is actually masculine or feminine. Unfortunately, you have to memorize the gender of each word individually. There are some patterns, but not many, and there isn't a pattern covering every noun.
Die - used for feminine nouns and plural nouns Der - used for masculine nouns Das - used for neuter nouns
Note that this is just for the nominative case.