- devDependencies are things like `babel-cli` and `prettier` or `ava` that you need to develop your app. If it is a command line tool or something that makes development possible, it should be here.
- install using `npm install <package> --dev` or `npm install <package> --save-dev` to add to package.json in devDependencies

- dependencies are packages that you are going to ship with the final product, and are required for your user to run your app, service or product.
- install using `npm install <package>` or `npm install <package> --save` to add to package.json in dependencies

- sometimes dependencies can also be devDependencies. Lodash is a common example of a package that is commonly used for development code (think unit tests, configuration scripts and similar) as well as front end.