Tempering is the process of heating and cooling chocolate in a specific way to form the right kinds of crystals in the cocoa butter.  Wikipedia tells me there are six types of crystals that can be formed, all based on temperature.  The problem is that all crystals take time to form.  If we just melt our chocolate and let it sit, most of the crystals formed will make our chocolate soft, dull, melt easily,and won’t have that pleasing snap when broken.  If we melt all the crystals and then hold the temperature at a certain point where we’re forming the crystals we want, then voila!  We’ll end up with firm chocolate with a glossy finish and a resounding snap when broken.  Dan “the Chocolate Man” Schreiber has written about his tempering experiences on his blog, where you can see what ill- and well-tempered chocolate can look like.

There are a couple traditional ways to temper chocolate.  One is the “tabling” method, where you spread most of your chocolate on a smooth surface, like marble and spread it to cool to a given temperature.  You can watch a video of Dan doing this here, or click through some photos at the bottom of the post.

You can also use the “seed” method, where you melt a bunch of chocolate, then drop in a chunk of well-tempered chocolate.  You can read a full explanation of this method, including some photos at Cooking for Engineers.The idea here is that by dropping in the right crystals, it will encourage the melted chocolate to form the same “good” crystals.