官方文档只是简单的说：
--save: Package will appear in your dependencies.
--save-dev: Package will appear in your devDependencies.
真正的区别是：
--save-dev:
dev指的是development：This is useful when installing development-only packages, like grunt, gulp or your testing library. Thus, when you are distributing your code to production, these dependencies will not be available.
这就是说，devDependencies下列出的模块，是我们开发时用的，比如grunt-contrib-uglify, 我们用它来混淆js文件，它们不会被部署到生成环境。有一个方法可以很好的检验这一点，将node_modules删掉，然后npm install --production会发现，只会安装到dependencies。
以及其他方便我们开发的工具，比如angular, vue, sass等都只需要安装到devDependencies。
--save:
You will use this option when you want to save a package dependency for distribution. Item such as angularjs, or any other module that is required at run time by your program.
dependencies下的模块，是我们生产环境中需要的依赖。
参考链接http://imcodebased.com/npm-save-or-save-dev-which-one-to-use/