It is non-transposing, like most instruments. Some wind instruments, such as the clarinet and the trumpet play a different note to what it says in the music. These instruments come in different sizes, e.g. a B flat clarinet and an A clarinet. I quote from the Observer's Book of Music. "To simplify the technical difficulties ...the music is not written in the key in which it is intended to sound. The orchestral clarinet player has 2 instruments, B flat and A. When he sees the note C written in his music, he associates that note with certain fingering, which, when used on the B flat instrument, produces the note B flat, and on the A instrument produces the note A." In other words the B flat clarinet sounds a whole tone lower than what is written. The composer has to write the notes a tone higher than he actually wants. All I can say is, you probably get used to it!
A violin does not have pistons and valves, which is what causes the problem with clarinets, trumpets and French horns, so its music is written "at pitch".