In other words, it's a Minecraft clone with one of the key features that Minecraft can't yet boast: a fully-functioning API.
At the moment, Voxel is very simple and there isn't very much to actually do in it yet, but it already provides a tech demo and a solid framework that others can build upon. It's creator, developer Max Ogden, spent just three weeks building the basis of this project and right now several simple iterations of it exist on his site. There's the example of a Voxel forest, there's the small Voxel world populated by simple creatures and there's even a little bit of showing off going on in this experiment with portals.
Right now, Voxel doesn't exist as much more than these rudimentary demos, but because it's open-source and because it can be ported to other platforms (including smartphones) it could travel far and wide, collecting more and more contributors as it goes and potentially growing into something very, very big. Could this become a genuine rival to Minecraft? Naturally, only time will tell, but there's certainly nothing stopping contributors from getting involved right now and, if you're interested, that could include you.
Thanks to VentureBeat, who have exchanged a few words with Ogden.
Here's an update for clarity (and for the sake of one person's sanity): Although I've mentioned the API before, I should state that it's the Application Program Interface, and as our lovely commenter Vinsanity says, "It's where 'universal codes' are collected, so anyone can program
stuff easier." Thank you, Vinsanity!