Minecraft is an immensely popular and very addictive sandbox game where you can build almost anything using blocks of various sizes. It is very similar to putting your imagination to work when playing with the sand at beaches and building castles, human figures and more. There is no limit to what you can do with Minecraft if you let your imagination run wild.
Now there is another similar game available called Terasology. It is open-source, runs using JRE (Java runtime environment) and provides a large number of modules just like Minecraft. This is why some people are also calling Terasology a clone of Minecraft.
In order to install Terasology in your computer, you can download the TerasologyLauncher from its Github website and launch it. The game runs on Windows, Mac and Linux, so choose the download meant for your system. If JRE is not installed in your system, it will launch web browser to download it from Oracle servers.
After this all you have to do is select a game release (alpha, beta, etc) and the version number before you can hit the download button. Download will be finished in a few seconds and then you will be able to click on the launch button to launch Terasology.
Terasology offers single player mode and multi-player mode. Even though it is an open-source game and still in alpha release mode, there are many public multiplayer servers that you can join to enjoy this game. In both modes, you can play the game either in windowed mode or in full-screen mode. Minecraft users will not find much difference between Minecraft and Terasology. In fact, Terasology generated worlds are visually much more impressive than those of Minecraft. It makes use of your graphics card in a very good manner.
You can find downloads, modules and helpful instructions about Terasology at https://github.com/MovingBlocks/Terasology.