I've been using company mode, which is a great completion framework for years now, and in genreal, it's phenomenal. For a while, however, I've had the feeling that I'm not getting completion options at exactly the right frequency that I'd like. And a completion framework that's a bit sluggish or that won't offer completions that you'd expect is a drag. I dug in a bit, and got a much better, because of some poor configuration choices I'd made, and I thought I write up my configuration.
Backend Configuration Company allows for configurable backends, which are just functions that provide completions, many of which are provided in the main company package, but also provided by many third (fourth?) party packages. These backends, then, are in a list which is stored in the company-backends, that company uses to try and find completions. When you get to a moment when you might want to complete things, emacs+company iterate through this list and build a list of expansions.