look up some youtube lets plays on it.
it is basically a sandbox adventure game where you build stupid blocky houses and junk, whatever you want to build.
the free version is not like an adventure game, it is only a sandbox building game. you can explore, but there's no real point in doing anything outside of making pixel art or crazy buildings.
if you buy the game, you get more of an adventure/game experience. you have a health meter, the ability to craft items such as armor, boats, tools, and lots of other goodies, enemies (mobs) that are both annoying and fun to fight, more incentive to actually explore your world (resources since you can't just build anything you want), and a few more things, i guess.
if collecting things is your bag, i'd say it's worth it. exploring a big underground opening that turns into your home mine is some kind of zen experience, if that kind of thing clicks with you.
otherwise, the game is a piece of crap and you should avoid it like the plague. but it's fairly cheap (though not as cheap as it was/still should be), and you could spend your money on worse games.
tl;dr: free = build ****
$20 = collect ****, build ****, fight ****, craft ****, get updates with more ****.
i'm unsure if mods can be installed if you play using the free version, and i'm pretty sure multiplayer is also available/enabled with free version. i may have played it like once. big whoop.
Also the fact that even if you plan on just building stuff in a Sandbox style, the paid version has like 999x more blocks to build stuff with.