Gamemaster
The source off all your fun and sorrow in role-playing games. Often a dick. Various games refeer to the GM (Game Master) with different names: Dungeon Master, Referee, Storyteller, etc.
How do I GMd?
A lot of people come asking for advice on how to run a role-playing game, truth of the matter is a game master is born, but a good game master is made. Only experience, reading and knowing the group of people you play with will help you become really good. Different GMs have different aproaches, some improvise everything, others painstakingly prepare every map, encounter and NPC the players come across. This way of doing things rarely pays off as players have the custom of shiting all over your work because they decide to take one wrong turn or ignore one person that was supposed to put them on the right track you prepared for them and wander off in the oposite direction. Some GMs counter this by railroading their players which is generally seen as a bad way of doing things - when being railroaded the players feel like involountary spectators of the GM's personal fantasy movie, which usually sucks.
If you know what your players want you're one step closer to running a good game. Some just want to kick some goblins' arses, others want to get involved in the political intrigue at court, others don't really know what they want. Try to lead them on adventures that involve all the characters and give them all challenges that depend on what they do best. If you have a rougue in the party make sure to have some sneaking or trapfinding to be done, if you have a barbarian be sure there will be oportunity to kick some ass and so on. Talk to your players.
In the end there is really only one rule, rule 0, which states: Have fun. Implying everyone at the table. Make sure everything is moving forward, try to avoind stalling and monotony. If the players are really stuck just throw something at them, even ninjas. Keep things happening and everyone interested.
If people aren't excited or interested it's often better to pull out another game, switch GMs, watch a movie or just eat pizza and chat instead.