There isn't an explicit objective unless you want to try for the achievements.

The only real objective is to survive and build stuff you like. It's a bit like a videogame version of legos... only instead of a little brother coming to smash your stuff, you have creepers.

Want to build a vibrant village and level up all the trades of the villagers? Go for it.

Want to ignore villages altogether and do your own thing? Knock yourself out. Build that fortress of solitude.

Want to live in a grass-block hut and breed a million cows? That's an option.