I have a terrible itch, a compulsion of such epic proportions that I am writing this with only one hand. In this instance my cravings have led only to a throbbing index finger, but I feel no less shameful for indulging in this hedonistic fulfilment, even if it is only making a pixellated man dig his way through a two dimensional subterranean world.
Terraria is beautiful in it's simplicity. You dig downwards, find stuff (gold, copper, goldfish etc.), then bring it back to the surface to make other stuff (houses, armour, lava-filled waste disposal units etc.) Along the way you encounter various 'baddies' who attempt to stop you consecrating the earth in this manor, but like any good capitalist free from tree-hugging interferers you just hack them to pieces with your pickaxe or other weapon of choice and continue your quest for mineral wealth.
Digging can be a laborious process (natch), but what keeps you going is two things. Firstly, if you find just a bit more gold you will finally be able to make those gold trousers (or grieves, or whatever the nerd word is) you've been lusting after. Internally we all want gold pants really, don't we? I do anyway. Secondly, as you dig ever deeper you will discover caves with pots full of stuff, skeletons to pummel, and even golden chests full of loot. Useful loot, not crappy swords that are better at this but worse at that, leaving you standing there bewildered as to which might prove more useful (majority of RPGs, I'm looking at YOU!)
Terraria is a dangerous time-sink, bath and swimming pool. Worlds are randomly generated, but no less fascinating to explore because of that. The game can be hard, with lower levels under ground providing a much greater challenge, but greater rewards to boot. Like the best RPGs, with a bit more effort you know you will eventually prevail, but unlike the worst RPGs, the grinding comes with such a variety of incentives that if never feels like a chore.
Should your interest be piqued in Terraria I have some advice. Firstly, take your time. Don't rush to get the best gear. Explore at leisure. Secondly, don't be too tempted to use the wikis, as they can spoil the excitement of finding new combinations of items. Finally, go and fill your boots, bother the natives and click til your finger bleeds.