There's a strange paradox to running a game: you want your friends to have fun, but you also need them to focus on the game at hand so as to immerse themselves in the story.
A great DM knows when to let the players sit back and joke and laugh and goof off, and when to call their attention to the game and settle the players down. For more information about dungeons and dragons, you can check here now.
But how do you do this? What's the right balance to strive for, and how can you get your player's attention without seeming a bore or a tyrant?
The first thing you have to realize and always remember is that the DM sets the tone of the game. The players will take their cues from you, so that if you're laughing and joking, they'll understand that now is the time to laugh and joke as well and that they need not worry about being 'IC' because the DM isn't pushing the game.
If you are serious and focused they will realize otherwise; This game started again, and now is the time to work hard and get a job. The tone is also governed by the nature of the game you're running.
Ravenloft would demand a totally different atmosphere, and how you approach these games run will determine many tones.