What is the difference between "require" and "require-dev"?

In Composer , Grunt for example, it has require and require-dev . My question is what's the difference between them?

Example phpBB

"require": {
    "php": ">=5.3.3,<7.0",
    "lusitanian/oauth": "0.2.*",
    "symfony/config": "2.3.*",
    "symfony/console": "2.3.*",
    "symfony/dependency-injection": "2.3.*",
    "symfony/event-dispatcher": "2.3.*",
    "symfony/http-kernel": "2.3.*",
    "symfony/routing": "2.3.*",
    "symfony/yaml": "2.3.*",
    "twig/twig": "1.*"
},
"require-dev": {
    "fabpot/goutte": "1.0.*",
    "phing/phing": "2.4.*",
    "phpunit/dbunit": "1.3.*",
    "phpunit/phpunit": "4.1.*",
    "sami/sami": "1.*",
    "squizlabs/php_codesniffer": "2.*",
    "symfony/browser-kit": "2.3.*",
    "symfony/css-selector": "2.3.*",
    "symfony/debug": "2.3.*",
    "symfony/dom-crawler": "2.3.*",
    "symfony/filesystem": "2.3.*",
    "symfony/finder": "2.3.*",
    "symfony/http-foundation": "2.3.*",
    "symfony/process": "2.3.*"
}