What's the difference between require and require-dev? [duplicate]

The require-dev packages are packages that aren't necessary for your project to work and shouldn't be included in the production version of your project.

Typically, these are packages such as phpunit/phpunit that you would only use during development.

seems clear to me:

require

Lists packages required by this package. The package will not be installed unless those requirements can be met.

require-dev (root-only)

Lists packages required for developing this package ⁽¹⁾, or running tests, etc. The dev requirements of the root package only will be installed if install is run with --dev or if update is run without --no-dev.

http://getcomposer.org/doc/04-schema.md

1. the packages used to develop a package