use-package is an Emacs package which allows packages to be loaded declaratively. It's been around for ages and I've seen it used in other people's configurations, but I've only recently paid some real attention to it. I wish I'd learned how to use it sooner - it's really improved my Emacs config.
Let's look at an example:
(use-package magit :bind (("C-x g" . magit-status) ("C-x C-g" . magit-status)))