Easy: Do you know which one is a Freddy vs a Bonnie, etc?
Rule of thumb:
- Freddy the bear (he's usually brown)
- Bonnie the rabbit (he's usually blue or purple)
- Chica the chicken (she's usually yellow)
- Foxy the fox (he's usually reddish-brown)
...And almost every other character is just a variation of one of those, with a handful of exceptions. :)
You'll also want to note
Golden Freddy and
Springtrap: they're gold-colored versions of Freddy and Bonnie, respectively, and significant to the story in a few ways. They're among the very first, oldest animatronics, but also able to be used as suits, wearable by humans! Unfortunately, they're incredibly dangerous to wear. If their springlock mechanisms fail, the animatronic pieces would snap into place, which would be Super Bad News for anyone inside. Currently: Golden Freddy is basically a haunted phantom stuck in limp suit mode, and Spring Bonnie is generally referred to as (something)-trap (as in, Springtrap, Scraptrap, Burntrap, etc) because the Big Bad of the early games died in it and became what amounts to a fur-suited undead menace.
An "endo" (endoskeleton) is an animatronic skeleton -- a bot without a costume on.
There's also
Puppet the sock-monkey-looking mime-type marionette doll animatronic, and
Balloon Boy the child-looking saboteur animatronic that only ever laughs and says "hello" and "hi", both from FNaF 2.
And "(the)
Mangle" is a pink-and-white mess of a bot that was a Foxy and a second endo, but got cobbled together, and whether they're male or female is a constant ongoing joke, not just in the fandom, but in the games themselves... to the point that the series' creator referred to them as both a he and a she in the same paragraph, in the game Ultimate Custom Night.
Hard(er): In pretty much every game, there's a new themed set of these characters, sometimes more than one. Sometimes games include new characters altogether, like the Funtime line/series, in Sister Location (FNaF 5), which includes a Freddy with a Bonnie hand-puppet, a Foxy,
Ballora (a human-based ballerina),
Circus Baby (a giant child-looking girl with pigtails), and miniature minions for both of the latter two.
Ennard is another distinct character from Sister Location: a mess of wires and coils formed by the insides of the Funtimes, twisted up into a single humanoid amalgamation, wearing a clown mask.
Some lines/series of characters are phantoms, or nightmares, or hallucinations. Some aren't even diegetic, but are merely fun reskins for the players' entertainment. Others are, in-universe, actual animatronics.
And then there's
the Mimic which is a whole 'nother nefarious bot that's just been introduced in the latest games....
It's kind of a rabbit hole, and is likely best explored
in a wiki. :)