A package is a folder that can contain anything that is listed in these examples (and other thing that can have nothing to do with Sublime Text 3, such as a
gulpfile.js if you're automating some tasks).
You can install any package using the create Package Control.
Maybe you've seen that, when you install a package using package control, you only get a
.sublime-package in the
Install Packages folder... How come you don't get the code? In fact, a
.sublime-package is a
.zip file renamed. You can extract or preview some files inside a
.sublime-package using PackageResourceViewer.