Sketch Library is an ordinary Sketch document that contains special
Symbols. These special Symbols can be perceived as master Symbol
instances. You can easily tell these Symbols apart from ordinary
Symbols, because they have a custom icon in the Layer List, Sketch
Library Symbol Icon.
To add a library, navigate to Preferences > Libraries tab
Kits are libraries of UI elements, app templates, and style guides. They create a solid jumping point for a faster workflow and better results when designing.
You can download them and add the elements to your template/asset/libraries to use them.
These are documents that make creating an app fast and easy by having premade buttons, styles elements etc.
For example, a good one to use would be Google's Material Design. It contains various elements that make up layouts. The sticker sheet contains light and dark themes for the status bar, app bar, bottom toolbar, cards, dropdowns, search field, dividers, side nav, dialog, and floating action buttons.
This allows you, for example, to load up a template. You simply go to File → New from Template → Material Design — and you’ll see a wide set of components, icons, and layouts.