In addition to the general mods that are can be used in all regular versions of the game, users of Minecraft Forge loader can also access a wide array of mods that are accessible only via Forge client. That game version is not some special customized game client, but instead usually just an older version of the game that maintains compatibility with this modding client.
Minecraft Forge is a free modding utility that utilizes open-source API and loader to provide comprehensive compatibility between numerous community-created mods and the exact version of Minecraft: Java Edition that is downloadable straight through the Minecraft Forge loader.