This is here to help you learn about the different types of operatic baritones and basses. It's intended to educate listeners rather than singers.
The bass is the lowest natural male voice. His voice might extend up to F above middle C and down to low C depending on Fach. The sub-categories for baritone and bass are many, some very specialized, that a full discussion may prove impossible. This is proving to be difficult, a learning experience also for me. We will start with this set and see how far we get.
If there are more categories, we are ignoring them. I am now going to describe the sub-categories, but please be aware that the same singer may show up in different sub-categories. A role may also cross into more than one category. I have tried in selecting these examples to make sure that the singer is actually of the suggested sub-category.
This is a pleasant low sound, basically the voice of the average male. Sample roles are Papageno in The Magic Flute (Mozart), Marcello in La bohème (Puccini), Don Giovanni in Don Giovanni (Mozart), Figaro in The Barber of Seville (Rossini), etc. Here is Simon Keenleyside singing "Der Vogelfänger bin ich ja" from The Magic Flute.
Sometimes this Fach includes a sub-sub-category called Verdi baritone. For me the Verdi baritone defines the dramatic baritone and does not require a Fach of its own. This voice needs a full tone for its entire range and that special Verdi intensity. Sample roles are Rigoletto in Rigoletto (Verdi), Scarpia in Tosca (Puccini), Simon Boccanegra in Simon Boccanegra (Verdi), Escamillo in Carmen (Bizet), Conte di Luna in Il trovatore (Verdi), etc. Dmitri Hvorostovsky sings "Il balen" from Il Trovatore.
I'm calling him a dramatic baritone. He is for me the greatest of operatic idols: Leonard Warren singing "Cortigiani" from Rigoletto.
What is a bass-baritone? He is a baritone with a really full, rich low register. He might be lower than a baritone or he might not. It's the sound that matters. Examples of lyric bass-baritones are: Méphistophélès in Faust (Gounod), Leporello in Don Giovanni (Mozart), Figaro in The Marriage of Figaro (Mozart), Philip II in Don Carlos (Verdi), Escamillo in Carmen (Bizet), Porgy in Porgy and Bess (Gershwin), The 4 Villains in Les contes d'Hoffmann (Offenbach), etc.
René Pape sings "Le veau d'or" from Faust.
This is Ferruccio Furlanetto singing "Ella giammai m'amo" from Don Carlo.
I think this is a category invented by Wagner. He wanted a bass sound with the Dutchman in Der fliegende Holländer, Wotan/Der Wanderer in the Ring Cycle and Hans Sachs in Die Meistersinger von Nürnberg while generally ignoring the upward boundaries of the Fach.
This example is like nothing else in the world. It is a young (35?) Hans Hotter singing "Die Frist ist um" from the Flying Dutchman.
BassA bass needs to have full resonance on very low notes. The Fach may extend to below the bass clef staff. Examples of roles are The Grand Inquisitor in Don Carlo (Verdi), Sarastro in Die Zauberflöte (Mozart), Baron Ochs in Der Rosenkavalier (R. Strauss), Il Commendatore in Don Giovanni (Mozart), Hunding in Die Walküre (Wagner), etc.
Here is a wonderful example of a baritone and a bass singing in the same scene. The older man is Rigoletto, a baritone, and the younger man is Sparafucile, an assassin who sings bass, including a nice low F at the end. Željko Lučić (Rigoletto) and Štefan Kocán (Sparafucile).
This is Charon from Monteverdi's L'Orfeo sung by Paul Gérimon.
There is a whole category for men who sing only comic roles called buffo bass or basso buffo. This is a long tradition starting in Italy. They are always basses but not usually very distinguished. Here is a favorite.
Some roles for this voice are Don Magnifico in La Cenerentola (Rossini), Leporello in Don Giovanni (Mozart), Dottor Dulcamara in L'elisir d'amore (Donizetti), Rocco in Fidelio (Beethoven), etc.
See here for countertenors, here for sopranos, here for tenors, and here for mezzos and contraltos.