If you had the opportunity to play the previous installments of Dragon Age, you can import the world characteristics. Then, all of the decisions that you have made so far will be implemented in the game world. Dragon Age Keep is a web-based service. It allows you to create the backstory for the past events in an easy way: while listening to stories of the events from the past, at the key moments, you can intervene - at that point, a button appears that allows you to make the same decision as you did when playing the game.
Using the above tool is completely optional - if you do not import anything, the game world is set to default.