It’s a voxel engine. It’s not anything on it’s own, but it’s made to be extended with games and mods to become you want it to be. And if you play online, the server will send all of this to your client, so you don’t have to download everything it uses.

It’s also not a “clone” of Minecraft, it doesn’t try to be Minecraft. Well, one of the games people made for it does but the base game itself isn’t.

It’s been around for a long time, since around 2010.