So, I think an example here will help you understand.

Let's use NodeJS with Express and with pg-node. Let's say you have a database.

  1. You define an Express handler to handle a get request.
  2. In that handler, you connect to a database, which takes a callback
  3. You create a query on that connection, this takes a callback.
  4. You then write a function that feeds the query result rows into a CSV exporter, which takes a callback (because of course it does).

You now have a pyramid something like 5 levels deep. Callback hell is in fact a thing.

However, I've found that pipeline of promises almost completely solve that problem.