Original: If it is a transposing instrument, the note written as C sounds as the note of the instrument's transposition - on an E alto saxophone, that note sounds as a concert E, on an A clarinet, that note sounds as a concert A/O. The bassoon is an exception; it is not a transposing instrument, yet its "home" scale is F.
Simplified: Saxophones and most brass instruments are transposing instruments.