The point to Minecraft is to make a world that is completely yours.
It is an extremely flexible game that caters to however you want to play it.
If you love zombie/survival/RPG games, then you play on Survival mode and try to survive as much as you can whilst building a shelter and whatnot for yourself.
If you simply love creating building and such, then you play on Creative mode. Creative mode has more god-mode abilities so you can create things easier than on survival mode where you have to hand-make completely everything you use.
And let's not forgot all the mods and skin packs you can download that enhances your gameplay greatly. There are no limits to mods, you can install guns/tanks/fighter jets, you can install more NPC's, you can install wallpapers and more colored blocks. Then there are these big themed mods you can install. If you want a more Renaissance feel to the game, or a more futuristic look, or possibly a steampunk look.
There's absolutely no limits, which is why I love the living hell out of this game. Plus, it doesn't take much to run so ideally, anyone can play. Also, the creators update it quite frequently, so there's always new things to make and play with, which keeps the players entertained and not get bored of the same ol' thing.