voxel.js is a collection of projects which allows you to create Minecraft-like games in the browser. voxel.js requires a browser which has WebGL support, as the games created with Voxel.js required it to run.
The idea behind voxel.js was to create a framework which is not monolithic but rather composed of several modules. Modules can be installed using Node.js package manager NPM.
The core components of voxel.js are the voxel-engine which is required for any voxel.js games, voxel the logic that generates the voxel game world and voxel-mesh which takes data from the previously mentioned voxel module and turns it into a three.js mesh that can be displayed as a game.
voxel.js also has several add-ons to complement the above core modules. They range from add-ons which allow you to create simple NPC creatures, to add-ons which can generate a forest or debrit. You can check out voxel.jss website for more information.