ah my foolish friend, usaily the prson who makes the RPG sets the rules and keeps people in line, people play and have fun understanding certain limits and such

You create a character, and then write about his adventures in small parts. You interact with other characters, and create your own niche in the RPG.
Currently, the largest, and, imo, the best, is the 'Control Your Own Faction' RPG. In that one, you join up as a FL faction, and build ships, have wars and battles, etc. Right now, there is a huge war going between two alliences of factions.