I don't know who needs to hear this but Apes are Monkeys
I'm not saying people need to start calling apes monkeys or anything, but if you're the kind of person to say "that's not a monkey, that's an ape", you are technically wrong.
A lot of people say that "if it has a tail, it's a monkey. If it doesn't, it's an ape" but frankly that's getting into "featherless biped" territory.
This is a barbary macaque. If you don't look at its waist, you'll probably not at all think it's an ape - and you'd be right - it isn't an ape at all. However, barbary macaques don't have tails, though all of their relatives do.
So, what actually defines an ape?
It's mostly about the torso actually!
Apes are more defined by having a more wide and shallow torso, while other monkeys have a narrower and deeper torso. Also, apes have shoulder blades on the back rather than on the side, and a greater range of mobility of the shoulders.
These adaptations were likely originally to allow for brachiation (swinging between branches and trees using the arms), which is still the dominant form of movement in gibbons
OK, so if that's why apes and monkeys are different, why did you say that apes ARE monkeys?
Well, Simiiformes, the infraorder that contains the monkeys (including apes) is split into two parvorders: Platyrrhini and Catarrhini.
Here's an example of some Platyrrhines or "New World monkeys":
Now, here's some Catarrhines, or "Old World monkeys":
As you can see, apes are nestled within the Catarrhines. The differences between Platyrrhines and Catarrhines are:
Platyrrhines are native to the Americas, while Catarrhines are native to Africa and Asia (and formerly Europe)
Platyrrhines have more sideways-facing nostrils, while Catarrhines have more downward-facing nostrils
Platyrrhines have 12 premolars (3 on each side of each jaw), while Catarrhines have 8 (2 on each side of each jaw)
Platyrrhines are usually smaller, ranging between 14-70cm (5.5-28in) in adult size and 0.12-15kg (0.26-33lb) in adult weight while Catarrhines are usually larger, ranging between 34-195cm (13-77in) and 0.7-219kg (1.5-483lb) (this is just in terms of living species and not including humans)
Some platyrrhines (specifically spider monkeys, howler monkeys, woolly monkeys, and capuchins) have prehensile tails, while Catarrhines do not
Platyrrhines do not have opposable thumbs (with the exception of capuchins and squirrel monkeys, which have semi-opposable thumbs), while Catarrhines do (with the exception of colobuses, which lack thumbs entirely)
Most platyrrhines are dichromatic (seeing two wavelengths of light) (with the exception of howler monkeys, which are trichromatic, and trichromancy exists as a female-only recessive gene in other platyrrhines), while catarrhines are trichromatic (seeing three wavelengths of light)
If both New World and Old World monkeys are monkeys to you, then apes should be as well.
Barbary macaque: Errol Véla @ abounabat on iNaturalist
Black-and-gold howler monkey: Carlos Schmidtutz @ carlossch on iNaturalist
Central American squirrel monkey: Michelle Monge-Velazquez @ mich_croc on iNaturalist
Bald uakari: @ sea-kangaroo on iNaturalist
Black-striped capuchin: Marcos Halem Felix @ marcosfelix_birdscuiaba on iNaturalist
Central American squirrel monkey: Roger Woodruff @ rdwoodruff on iNaturalist
Formosan rock macaque: Jurga Motiejūnaitė @ jurga_li on iNaturalist
Mountain gorilla: Laura Keene @ laurakeene on iNaturalist
Eastern chimpanzee: Ad Konings @ adkonings on iNaturalist
Udzungwa red colobus & Yellow baboon: Zein et Carlo @ lesbectrotters on iNaturalist
Patas monkey: Amadou Bahleman Farid @ bahleman on iNaturalist