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$-25$.
You can find Diablo private server and Diablo MMORPG server - we have some of that Diablo private servers in our top mmorpg list you just see as top 100.
The precursor to MMORPGs were MUDs, short for "Multi-User Dungeons", which began in 1978. These were text-based games that allowed many users to simultaneously play, interact, and create pieces of their online environment. Graphical user environments came into being in the 1980s, but they were all on proprietary networks. In 1996, 3DO brought the first MMORPG, called Meridian 59, to the Internet. With a limit of 250 users at any time, it was not "massive" by today's standards, but was the largest game of its kind to date. The first successful MMORPG was Ultima Online, a fantasy game based on the Ultima series of role playing games.
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. 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. 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).