When use npm to manage package, will use --save and --save-dev, this topic used vuex as example to show the different

{ 1. npm install vuex }

In short it's one time install, will not change package.json, if you removed the package can not reinstall with npm install command, so only use it for no need repeat package

{ 2. npm install vuex --save }

this command will change package.json, and add vuex to "dependencies" section, usually use this command to install core package for web app run, like: vue, vue-router

{ 3. npm install vuex --save-dev }

this command will change package.json, and add vuex to "devDependencies", usually use this command to install dev support package, like: file-loader, webpack