What is Generation Stage Meditation also known as Utpattikrama?

There are many forms of meditation found in Buddhism. Zazen is possibly the most well know form of meditation in the West. However, in there are many different types of mediation. In Vajrayana there appears to be many different types and stages of mediation as well. One form is called "generation stage meditation" or also known as utpattikrama. What exactly is "generation stage meditation" and how is it practiced?