If you've never heard of this game, let me try to explain: it's an exploration game, similar to the more popular/well-known game, Minecraft. You can mine ores in the ground, use them to craft new tools, weapons, buildings and such. There are enemies in the world, from simple slimes to massive bosses, to interact with along the way.
The fun of the game comes from the random exploration aspect of it; the advancement of your tools and the discovery of new biomes (from the desert, to the jungle... the caverns to the underworld... and even oceans and floating islands in the sky). As you grow and explore deeper and further into the world, you can improve your stats and gear, meet new NPCs who will help you out on your adventure, find new cosmetic items or accessories, learn new crafting plans. And now (well, tomorrow...) there will be much, much more of this to go around.
At $10 on STEAM, this game is a steal. On sale (the recently concluded "Autumn Sale" or the upcoming "Winter/Holiday Sale"), you can find it for as little as $5, which makes it an absolute must-own in my books. Getting started can seem a little daunting and perhaps a bit un-structured, however, the official Terraria Online Wiki is an excellent resource.
» Found at: Terraria Online (DevBlog)