Ahh, someone finally made a thread.
It's probably one of the most amazing games I've ever played.
During the day, it's a building/sandbox game. You can do whatever you want. Collect resources, build houses, craft materials and weapons, dig, and like the game says, mine. Literally whatever. During the night in the game, however, it's more of a survival game. These creepers come out and will kill you. There's giant spiders, skeletons with arrows, zombies and flaming zombies. Now, if you made a shelter during the day like you were supposed to, then you'd be fine.
The best part is exploring underground. If you get lucky, you'll come across a series of underground caverns that take literally hours to explore.
Here's a good series of videos if you want to watch.