This is what I do.
I've been running a game consistently, every Friday (more or less, we tend to take the holidays off, and things come up) since I was eleven years old. Yes, I kid you not, I've been doing this for over thirty years. I don't play much D&D anymore. Steve Jackson's GURPS, White wolf's Mage the ascension. Champions, and Paranoia are the things I run, pretty much in order of competence. I've played in so many I can't list them all.
Right now I'm running a GURPS game, my hodgepodge world I've built over twenty odd years. I'm blogging about it here. The site is not well organized yet, but I will fiddle with it as I learn, until it is far more presentable.
I'm usually the game master, (also called dungeon master, story teller, or controller) I set the stage, prepare the backgrounds, create the story, and keep the group more or less on track. The players then interact with what I give them to (hopefully) have a fun adventure.
I'd say I've ran a game of one sort or another with well over a hundred people. I think of them all fondly.
Now we've gotten pretty organized on the whole thing, mostly due to my wifes love of structure. We have a potluck before hand, where someone provides a main dish, and the rest accessorize. Sessions tend to last around four hours, and we all have a pretty good time. She sets up a google wave with all the current roster and the organization sort of takes care of itself
The longest game I've run lasted about three years doing it this way, but most go about six months. I tend to try and tell stories, with a beginning, a middle, and an end. Sometimes the NEXT game will be run with the player characters of the older games as NPC's, sometimes as helpers, sometimes as adversaries