A singer who is a bass is a man with a low singing voice. Some basses may be able to sing down to C two octaves below middle C. Even low A is sometimes needed in Russian church music.
In opera basses often have the part of the villain, or the king, or a priest. A buffo-bass is a character role in which the bass singer needs a wide range for the comedy of the part. The part of Leporello in Mozart's Don Giovanni needs a buffo bass. The lowest kind of bass is called a basso-profondo. An example is the priest Sarastro in Mozart's Magic Flute.
Feodor Chaliapin (1873-1938) was a famous Russian bass.
A baritone is a voice between tenor and bass in range, but some low baritones describe themselves as bass-baritones. Wotan in Der Ring des Nibelungen is a bass-baritone part. Bryn Terfel is a famous Welsh bass-baritone.