Some instruments are what we call transposing instruments. For various historical reasons, these instruments are written in keys different than how they sound. A good rule to remember is:
When a transposing instrument plays a written C, it sounds its name.
In other words, when a Horn in F plays a written C, it sounds its name, or F. From this we realize that the Horn in F sounds a perfect fifth lower than it is written. So these horns that start off on a written E♭ are actually sounding the A♭ below it.
The Clarinets in B are actually in B♭. This is because of the German tradition that B is B♭ and H is B♮. But the rule is still the same: when a B♭ Clarinet plays a written C, it sounds a B♭. Since this is a major second lower than written, we know that their written B♭ sounds like an A♭.
So, a little test for you, if you want (put your cursor over the blank answer): if a Trumpet in D plays an E♭, what pitch will it sound like?
When it comes to the timpani (Pauken), each individual drum is just tuned to one of those three pitches. In other words, the head of one drum is stretched such that hitting it produces an F; the other, a C; and the last, an A.