A-List: A hero who is capable of sustaining their own series for an extended period of time (possibly more than one series, but at least one.) If they guest star, they are given major billing on the cover and their presence is usually intended to be a sales boost to the title. May be known to the general public, although this is not a requirement. They are almost immune to comic book death; if they do die, their books do not cease publication and the character returns from death in no more than five years. Examples: Superman, Green Lantern, Spider-Man, Captain America.
B-List: A frequent recurring hero in their fictional universe, someone who is usually a regular in an ensemble book. They may or may not have gotten their own series, but usually have not been able to sustain one for more than about five years. They are generally popular, to the point where a casual observer might think them to be more popular than an A-list hero simply because of the clamor for them to get their own book. They make guest appearances on a fairly regular basis in other books, but are not always billed as such on the cover. They are relatively safe from comic book death, but they can die in major crossovers; however, they usually return within five years as well. Examples: Martian Manhunter, the Thing, the Beast, Guy Gardner.
C-List: These heroes usually enjoyed a brief spate of popularity at the beginning of their existence, only to fall out of favor with the comic reading public. They rarely appear in their fictional universe, and when they do, as often as not it’s simply to provide a “shocking” death in a major crossover. (C-list characters’ deaths are almost always permanent; if they do return, it’s most often with a different character bearing the same name/costume.) They are highly unlikely to get their own book or even to make regular appearances in other books; while they have their fans, they’re generally considered “cult” characters at best. Examples: Speedball/Penance, War Machine, Vigilante, Judomaster.
A-List: These villains have lofty goals (world domination, destruction of the universe, unmitigated chaos) and the ability to achieve them. Their appearance is considered a significant event every time they show up, usually an occasion for serious difficulties for the hero and a major extended plotline is needed to defeat them. Heroes sometimes need help against A-list villains, and they can be the headliners of crossovers (but not always.) Their defeats will be less ignominious than other villains, and they may even have a few victories notched up on their belt. (Sometimes they wind up having their defeats explained away as the work of imposters, clones, or similar.) If they do die, it’s almost guaranteed that they will return from that state. Examples: Doctor Doom, Darkseid, Thanos, Lex Luthor.
B-List: These villains tend to have less interesting, more self-motivated goals (personal wealth and power, revenge against the hero or society, et cetera) and are of a lower level of power. They are still significant threats to the hero (although they become less so over time–it’s more likely for a B-list villain to become C-list than A-list) and they generally remain thematically tied to a single hero. They put up a good fight most of the time, and are still exciting to read about, but generally more because of their particular quirks and personality than because they present a major obstacle to the hero’s life. They are not immune to comic book death, but it’s generally presented as a major event when they die. Examples: Kraven the Hunter, the Abomination, Metallo, the Parasite.
C-List: These villains are usually hired out or serve as lackeys to more powerful villains; regardless of who they originally battled against, they now make appearances in any book where a “disposable” villain is called for. They do not present a significant threat to whatever hero they face, and indeed they usually wind up defeated in comically ignominious fashion; their power levels may or may not be low, but if they are powerful, they are not skilled enough to use their powers in a threatening fashion. They are frequently the targets of “edgy” heroes or other villains, who kill them to show how tough/uncompromising they are. If they are killed, as with C-list heroes, someone else usually winds up taking on their mantle. Examples: Plantman, the Constrictor, Terra-Man, Weather Wizard.