What does -save-dev mean in npm install grunt --save-dev

There are (at least) two types of package dependencies you can indicate in your package.json files:

1. Those packages that are required in order to use your module are listed under the "dependencies" property. Using npm you can add those dependencies to your package.json file this way:

   npm install --save packageName
   

2. Those packages required in order to help develop your module are listed under the "devDependencies" property. These packages are not necessary for others to use the module, but if they want to help develop the module, these packages will be needed. Using npm you can add those devDependencies to your package.json file this way:

   npm install --save-dev packageName
   

To add on to Andreas' answer, you can install only the dependencies by using:

npm install --production

--save-dev: Package will appear in your devDependencies.

According to the npm install docs.

If someone is planning on downloading and using your module in their program, then they probably don't want or need to download and build the external test or documentation framework that you use.

In other words, when you run npm install, your project's devDependencies will be installed, but the devDependencies for any packages that your app depends on will not be installed; further, other apps having your app as a dependency need not install your devDependencies. Such modules should only be needed when developing the app (eg grunt, mocha etc).

According to the package.json docs

Edit: Attempt at visualising what npm install does:

• yourproject
  • dependency installed
    • dependency installed
      • dependency installed
      • devDependency NOT installed
    • devDependency NOT installed
  • devDependency installed
    • dependency installed
    • devDependency NOT installed

When you use the parameter "--save" your dependency will go inside the #1 below in package.json. When you use the parameter "--save-dev" your dependency will go inside the #2 below in package.json.

#1. "dependencies": these packages are required by your application in production.

#2. "devDependencies": these packages are only needed for development and testing