What's the difference between require and require-dev?

I would like to know the difference between `require` and `require-dev`. The composer website doesn't offer a good explanation the difference between these two. The part that I don't get is `Lists packages required for developing this package, or running tests, etc.` from [Composer Official Docs.][1] [1]: http://getcomposer.org/doc/04-schema.md#package-links