What are the different Composer package types in Magento 2?

Magento system uses Composer packages to distribute, install, and upgrade components in an application.

Table below shows all the package type used in the Magento.

Friendly name	composer.json `type`
Metapackage	metapackage
Module	magento2-module
Theme	magento2-theme
Language package	magento2-language

Sample composer.json from Magento system as follows :

Metapackage

Technically, a Composer package type, not a Magento component type. A metapackage consists of only a composer.json file that specifies a list of components and their dependencies. For example, both Magento Open Source and Magento Commerce are metapackages. (Reference Magento DevDocs)

{
    "name": "sudhanshu/module-main",
    "version": "1.0.0",
    "type": "metapackage",
    "require": {
        "sudhanshu/module-abc": "1.0.0",
        "sudhanshu/module-xyz": "1.0.0"      
    }
}

Module Package

/vendor/magento/module-msrp/composer.json file for module

{
    "name": "magento/module-msrp",
    "description": "N/A",
    "require": {
        "php": "~7.0.13|~7.1.0",
        "magento/module-store": "100.2.*",
        "magento/module-catalog": "102.0.*",
        "magento/module-downloadable": "100.2.*",
        "magento/module-eav": "101.0.*",
        "magento/module-grouped-product": "100.2.*",
        "magento/module-tax": "100.2.*",
        "magento/framework": "101.0.*"
    },
    "suggest": {
        "magento/module-bundle": "100.2.*",
        "magento/module-msrp-sample-data": "Sample Data version:100.2.*"
    },
    "type": "magento2-module",
    "version": "100.2.3",
    "license": [
        "OSL-3.0",
        "AFL-3.0"
    ],
    "autoload": {
        "files": [
            "registration.php"
        ],
        "psr-4": {
            "Magento\\Msrp\\": ""
        }
    }
}

Theme Package

/vendor/magento/theme-frontend-blank/composer.json for theme

{
    "name": "magento/theme-frontend-blank",
    "description": "N/A",
    "require": {
        "php": "~7.0.13|~7.1.0",
        "magento/framework": "101.0.*"
    },
    "type": "magento2-theme",
    "version": "100.2.5",
    "license": [
        "OSL-3.0",
        "AFL-3.0"
    ],
    "autoload": {
        "files": [
            "registration.php"
        ]
    }
}

Language Package

/vendor/magento/language-en_us/composer.json for language

{
    "name": "magento/language-en_us",
    "description": "English (United States) language",
    "version": "100.2.0",
    "license": [
        "OSL-3.0",
        "AFL-3.0"
    ],
    "require": {
        "magento/framework": "101.0.*"
    },
    "type": "magento2-language",
    "autoload": {
        "files": [
            "registration.php"
        ]
    }
}