Q: What is the difference between a commercial and a free mmorpg ?

A: a free mmorpg means that you do not have to pay for play. A commercial mmorpg like world of warcraft will cost you a monthy fee between 10$-25$. The influence of time spent on rewards and thus social status in MMORPGs has led to a curious reversal of how people regard time spent: In other forms of entertainment the time spent in the entertainment activity is a gain, in a MMORPG time spent is often considered a loss, a cost. If you paid $15 for a movie ticket, you'd be seriously annoyed if the movie lasted only 5 minutes, because you counted on having paid for something like 90 minutes of entertainment. In MMORPGs, if it would take 90 minutes of killing monsters to do a quest and get a reward instead of just 5 minutes, you'd complain about "the grind". Any time spent in a MMORPG in an activity that doesn't give a reward is considered pointless, and any addition of a reward even as silly as an "achievement" to a previously pointless activity will make players pursue it.

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Massively multiplayer online role-playing game (MMORPG) is a genre of computer role-playing games in which a very large number of players interact with one another within a virtual game world.
As in all RPGs, players assume the role of a fictional character (often in a fantasy world), and take control over many of that characters actions.

A free mmorpg works like a free game. You can play for free but often you can buy items in a so called item shop.
MMORPGs are distinguished from single-player or small multi-player RPGs by the number of players, and by the game persistent world, usually hosted by the game publisher, which continues to exist and evolve while the player is away from the game (afk).