The design systems we create for clients at akanoodles usually consist of: Typography principles, Grid systems, Colours palettes, Spacing laws, Iconography, Button logic and of course a Pattern library.
Pattern libraries do not appear overnight and take effort. But a well planned and maintained pattern library will ensure a consistent user interface, facilitate reusability and make maintenance easier.
What a pattern library does it documents all of the modules used on your site and defines how they behave, what they look like and how they are coded.
When working on a pattern library we tend to put the skeleton together as we design the site. Starting off with wireframes of individual patterns, notes on how that pattern works and other considerations. In the first instance, they act as a functional specification for developers so it's a good way of organising all your documentation and it looks like you know what you're doing!
Here is an examples of a library we have contributed to: 'CNN Playbook'