Its like Minecraft, but in my opinion, much better. Its a little less about building crazy structures and more about exploring and stuff, watch gameplay of it, it also has a very good physics engine. (For a 16 MB game its amazing.)
*Trying to keep the piracy talk to a minimum*
Also, you're supposed to buy it on steam.( I bought it because I like to support developers) But you don't need to have to have a legit copy to play online.
You just have to enter the IP of my server through the multiplayer options. (Right now im using Hamachi, not portforwarding)