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one is the intuitive "a set is a collection of distinct objects" like {dog, cat, cow}. this is where a lot of people trying to introduce sets to somebody will start. but if you start actually reading any set theory you run into a much more confusing concept of set

in ZFC set theory (which supposedly provides the foundation for modern mathematics) sets don't have "ur-elements" - there's no "dog" or "cat" or w/e. the only things a set in set theory can contain are *other sets*. so you start with the empty set and build up from there

this is what you could call an implementation detail in how mathematical objects are created in ZFC, roughly analogous to how everything on a digital computer is ultimately strings of 0's and 1's. most of the time this is deliberately hidden behind an abstraction layer

we want to be able to use sets to produce collections of mathematical objects, e.g. the set of natural numbers {1, 2, 3, ...}. but each of those numbers 1, 2, 3, as an object in ZFC, must *itself* be a set! which set is it???

in some sense it doesn't matter as long as you pick a convention and stick with it. the standard construction is actually very cute - 0 is the empty set ∅, 1 is the set {0} = {∅}, 2 is the set {0, 1} = {∅, {∅}}, 3 is the set {0, 1, 2} = {∅, {∅}, {∅, {∅}}}, etc.

stuff gets more confusing in an annoying way once you start asking about more complicated mathematical objects. e.g. we could talk about sets of real numbers like {0.7, e, pi}. which sets are those??? here you have to pick a convention about how to describe real numbers as sets

this can be done but imo it's tedious annoying stuff and people who are not set theorists do it at most once and forget about it as soon as possible b/c as long as the interface works properly you can ignore all the implementation details

the "interface" in this case being stuff like, you can add real numbers, you can multiply them, etc. etc. as long as that stuff is all working as it should be totally ignore the fact that real numbers are technically sets, in practice

this is something i wish people talked about more - in practice math runs on what you could call a "high-level programming language" and set theory dives into the "low-level programming language" underneath it that most mathematicians would rather ignore