The bassoon is tuned to C and covers about 3.5 octaves, ranging from low B-flat to high E above the staff. So, the bassoon's pitch spans from bass-baritone to tenor range.
The diagram illustrates the airflow (in red) with all the tone holes and keys closed.
The bassoon is notated using two clefs: the bass clef and the tenor clef. The tenor clef is used when the notation extends far beyond the staff. The musical example makes this difference pretty clear.
After a short adjustment period, newcomers to the bassoon won't see switching between the clefs as a burden but rather as a huge help.