There are tons of things to craft in Terraria from basic iron tools, to epic molten armour which regenerates your mana, and yes you have mana! you can use it to fly, cast spells and use magic!
To craft most of the awesome things though, you need to work your way down into the earth, with a pickaxe, you can mine for miles downwards in the huge world the is randomly generated, you will find all different ores down there and lots of monsters, so be careful!
And when you think you are ready, you can summon massive bosses like the giant Eye of Cthulhu, but the best thing about the game is, its multi player feature, the game is so fun on multi player especially if you use a voice chat software like team speak.
Here is a demo of the game:
And here is the link to the website if you are interested in buying it: http://www.terraria.org/