Minecraft is basically...uh..minecrafting. A random world is generated and you are dropped into it to survive. You do this by digging up the earth and harvesting plants and then crafting them into successively cooler stuff in order to give you a survival chance against the weird creatures that populate the world and tend to come out at night to fuck your shit up. You can also opt to turn off the weird creatures and just simply explore and dig and build without any threats (other than hurting yourself by falling). That's the basic summary. Most of the videos you see on YouTube where the person built something huge and insane were done in the old version of Minecraft where you could simply generate bricks out of thin air without having to craft or dig.