How do MMO private servers work?

so I was talking to this one guy about World of Warcraft. I told him that I didn't really like the game and besides there is no way I was going to pay monthly for that. He was telling me that he doesn't pay monthly. He plays on a free private server. I had not heard of that so I tried to get on one for various pay to play games and I have no Idea how that actually works. The sites talking about these private servers. All say you have to register and then once you do they say ok now you can download it and play. But that doesn't make any sense. All I have done is just got the original version of the game one that still requires me to pay for it. They don't even have any download links or anything on their sites. How do people actually get into these private servers?

In my experience, private servers are just previous and usually never current version of games you could play.
The problem with is, that its illegal. So, more often than that, by the time you heart about them, they're shut down already.

I wouldn't suggest setting one up either as the original company can sue you out the ass. This doesn't happen with all games though.

Pretty much this, wouldn't recommend using or making one.

Lol@the idiots actually playing on those shitty servers.

There is one private server that has been around for a good bit. It's literally Vanilla WoW, the patch before Burning crusade, but besides that most of them try to do their own thing.

As said above, I'd advise against them. Very...VERY few are even decent and most of the time that's a maybe.

It's actually not easy to keep a private server. Your not the one paying but, he owner actually does. If he doesn't get enough money to support the server, it's game over.
You need a lot of people to actually make a private server. Imagine scripting all of those NPC by your self? Keeping every one happy by making events. A owner must keep his server interesting or else no one will play.
But in my thinking, scripting is the hardest. And imagine doing that in world of war craft... That's F load of NPCs.

You know how server<->client relation in networking works? Private servers take the data from the game and run it down in their own scripts - that's where the problem with quality comes in. Blizzard isn't handing out their server software so people have to re-produce it manually. It lets you connect as a regular client and executes the game down in the usual manner, that's why you have to use the original client of the game since that's the whole idea behind it. Think of it like how you would switch from one official server to the other, just that it's run on a different software. That's why the private server hands you out custom files or instructions on how to change the way your local game client gathers server information so their own stuff shows up on the list.

The quality of private servers is quite game dependant. I've played a certain MMO just as long on private servers as I did on the officials, and there weren't any issues - often more support and more server stability than seen on the officials. On most engines scripting the way AI works (Pathing, range detection and/or the trigger of projectile usage that comes with it, various fighting algorhytms like directional turning.) is a huge fuckton of work to reproduce, thats why, on WoW servers for example, the quality of gameplay just suffers greatly due to all the PvE and pet-based gameplay involved. Other games are just so simple in terms of mechanics that reproducing them isn't all that of an issue after all, which is the case in most really old games.