Basically you build yourself a castle and explore the world.
battle enemies, upgrade your gear. and dig your time away.
Yo can also join other servers, and participate with them there...check out their wiki, pretty extensive.
Love M&B, lots of mods/support as well, which is great and prolongs the game's life. Settings can be tweaked to get performance on an ailing machine or massive cinematic battles on a flagship tower.
Minecraft is surprisingly entertaining, though there isn't really an endgame (as of now). Basically explore, mine, build, survive