Its a massive hit for a reason, play it and hate your self for not tiring it sooner.
Starting out this games gives you absolutely no instructions on how things work, no story, no goals, nothing. It just spawns you in some random location in this huge map where everything is made out of blocks. Each block is made out of something like in the real world, sand,dirt,wood,rock, ect. You can then hit/mine these blocks till they break and you pick them up and can now place these block were ever and how ever you wish.
certain blocks can be made into other useful things and then those things can be combined with other blocks to made more things. All theses things combine in a very semi realistic way that can be mostly found out by experimenting with the blocks and the crafting option but many of the more advanced items to craft are very hard to just stumble on. Many times you'll find your self going online to search how to craft new items.
My recommendation to anyone wondering what the big deal is is just play it a few times and you'll understand.
The map is huge not just in its size in how far you can see but how deep it is. when you start to mine down into the earth you start to find caves dungeons and then your start to realize just how big the map truly is.
Before I actually played this game I couldn't understand why people loved this game so much but after playing it and on more than one occasion have lost track of time only to spend hours playing it, I finally understand.