There are four classes of monster in Dark Summoner that correspond to the three Guilds: Impulse (Sin Guild), Covert (Rancor Guild) and Psycho (Tyranny Guild) and a separate class with no Guild, Cypher. They are arranged into the 16 tiers C, C+, B, B+, A, A+, Ex, AA, AA+, AAA, AAA+, S, S+ and G (God) with C being the most common and G the rarest. Each Rank, with the exception of Ex & G, has a base level form and an evolved form indicated by a plus (+) sign. Rank Ex monsters evolve differently and have four unique evolution stages with an option to select from five different skills when reaching the fourth stage, while G doesn't have any evolution since it's the highest evolution of some S+. Also see Leveling cost, Evolution.
Currently there are over 2500 monsters to be found. Over 800 monsters in each of the main guilds, and over 50 that are cypher.
There are now also S and S+ monsters. Whoever made this page hasn't updated it in quite some time. I'm not an editor and will not fix it.
Someone needs to update to add S and S+ monster stats please. I would but I don't know them all.