It's not quite an MMORPG in the sense that WoW is (you can party with a max of 6 people, counting yourself, at a time), but it's definitely massively multiplayer. It's primarily an extension and refinement of D2, if you're familiar with that game. Trading is pretty strongly emphasized (though there's an entire section of the game for people who want to go it alone, known as Solo Self-Found), and a lot of the best gear is near-impossible to get without trading, and in many cases still quite difficult to get even with trading, as some of the best items cost truly absurd amounts of in-game currency to buy.

It is F2P, but it's difficult to manage with the extremely limited stash space you have available without paying. You will also find that your characters tend to look pretty bad without what the game calls "microtransactions," which are basically skins for various aspects of your character.

It's also quite complex; there's a ton of stuff going on in the game, which can be daunting to confront all at once. A lot of it can be simply ignored, and indeed most players have a specific set of content they primarily engage with, and another set they largely or entirely ignore. Other parts are inescapable, such as the very large passive-skill tree, which tends to frighten off a lot of potential players (but most of that can also be ignored, by simply following builds that tell you what parts of the tree to spend points on and when).

Now, with all of that said, it's a lot of fun if you can get into it, and the game is definitely designed to absorb as much time as you're willing to spend on it. Top-tier items are difficult to get, but not impossible if you learn the ropes, and those items are considered top-tier generally because they're either ridiculously powerful and/or insanely fun to use. (Anyone who's ever had a chance to use a Headhunter belt, one such top-tier item, can definitely vouch for this.)