Tempering Chocolate: Tempering is important because it determines the final gloss, hardness, and contraction of the chocolate. When you melt chocolate, the molecules of fat separate. In order to put them back together, you temper it. There are a variety of ways to do it.
One of the easiest ways to temper chocolate is to place it in the microwave for 30 seconds at a time on high power until the chocolate is melted. Be very careful not to overheat it. The chocolate should be only slightly warmer than your bottom lip, and it will retain its shape even when mostly melted. Any remaining lumps will melt in the chocolate's residual heat. You can also use an immersion blender to break up the lumps and start the recrystallization process. Usually, the chocolate begins to set, or crystallize, along the side of the bowl. As it sets, mix those crystals into the melted chocolate to temper it. I like to use a glass bowl because it retains the heat and keeps the chocolate tempered longer.
Another way to temper chocolate is called seeding. In this method, tempering is achieved by adding small pieces of unmelted chocolate to melted chocolate. The amount of unmelted chocolate to be added depends on the temperature of the melted chocolate but is usually 1/4 of the total amount. I usually use an immersion blender to mix them together.
From Dessert Circus, Extraordinary Desserts You Can Make At Home by Jacques Torres