They're simple.

The reason you're confused is because JavaScript lacks explicitly written types and you don't know closures and variable life cycles.

Think of it this way, when a function that takes a callback is invoked/called then that entire callback/function definition now lives within the called function.

I.e.

function derp() { return 1; }

function horse(cb) {

return 1 + cb() ; }

function horse2() { function cb() { return 1; } return 1 + cb() ; }

If you see the example above, in the first scenario you have function horse that accepts a callback which makes it more versatile and in horse2 you see the same cb function is defined internally but you have the same result.

Hopefully this helps.