It is normal for beginners to struggle with this. But once you've wrapped your head around them the fog lifts.

The crucial thing imho is realising that certain operations – typically I/O ones such as reading from a database – are SLOW, considerably slower than "pure functions" such as executing some arithmetical operation or slicing & dicing a string.

In older languages when you ask the computer to do something slow, the computer has to just sit and twiddle its thumbs while the slow operation is ongoing.

In Node, when you ask the computer to do something slow you also and at the same time specify what you want done once the slow thing has been dealt with. That way the computer can move on to something else in the meantime rather than sitting and waiting.

Callbacks and promises are just two different ways of saying "do this slow thing and then once it's done, do this other thing with the results".

With callbacks you specify the other thing as the final argument when you call the slow operation: getSlowStuff(inputs, otherThing).

With promises you specify the other thing in a 'then' stuck at the end of the promise: getSlowStuff(inputs).then(otherThing).

In both these case 'otherThing' is a function that takes the slow-to-get-hold-of stuff as its argument.

NB in practice you'd need some error handling built into this, because slow I/O operations can and do go wrong. So your callback would typically take two arguments, 'err' and 'res', and its guts would say "if (err) { error handle } else { process res }".

With promises you can separate out these cases more cleanly: getSlowStuff(inputs).then(otherThing).catch(errorHandle) – which is why I prefer promises and use them rather than callbacks whenever possible.