its like minecraft... in that you dig to build. (you build new tools out of better materials, which makes them do more damage and lets them work faster.)
you dont actually have to dig to explore caves though, you can actually find them relatively easy on the surface(givin that its 2d). caves are filled with monsters and pots(with items in them) and treasure chests (with items in them). oh there are also floating islands with special items and stuff on em.
you've got npcs that you can build houses to make em stay in "town"(i use that word loosely) guide, merchant, nurse, arms dealer, etc.)
youve got a random dungeon on each world, and an underground jungle.
corrupted lands that have shadow orbs you break to find rare items and summon bosses/meteors.
with a little bit of Wow tier gear thrown in. everything after gold gives you set bonuses.
oh, god bless the grappling hook.
yes... im an addict.