The gameplay is mostly sandbox-style. There are no monsters or animals. You start in a blocky world somewhere between the coldest glacier and the hottest desert and just do whatever: Craft a pickaxe and go mining for ores, gather blocks and resources, craft new blocks, tools and items, go farming, use your new blocks build an awesome house, and so on. Read the Minetest Game wiki page for a full explanation.
Minetest Game
A relatively minimal game for modding. Customize your world to your liking.