At its core Dungeons & Dragons is a role-playing game. When you put aside all the mechanics, character classes and dice rolling the game is about the story. The story is usually narrated by the DM who explains what transpires. The DM is literally responsible for everything the PCs experience. The DM is also responsible for shaping the direction of the story. From planning encounters, designing skill challenges and writing the adventures the DM fills some pretty big shoes.
One of the most important decisions a DM makes is choosing which mode of storytelling they want to adopt for a particular campaign. Do you use existing adventures from Dungeon magazine or Living Forgotten Realms (LFR) modules? Do you create your own adventures and storyline. How much choice will you allow the PCs to have? Will you railroad the PCs down the path you want the adventure to progress in or will you allow the PCs any choice they wish and run a sandbox campaign? Why not a little bit of both?