Clarified butter is butter that has had the milk solids and water removed. One advantage of clarified butter is that it has a much higher smoke point, so you can cook with it at higher temperatures without it browning and burning. Also, without the milk solids, clarified butter can be kept for much longer without going rancid.
It is very easy to make. Melt the butter slowly. Let it sit for a bit to separate. Skim off the foam that rises to the top, and gently pour the butter off of the milk solids, which have settled to the bottom. A stick, 8 tablespoons, of butter will produce about 6 tablespoons of clarified butter.
Another method is to simmer the butter in a saucepan until the mixture separates. After the water has evaporated, the milk solids will begin to fry in the clear butterfat. When they begin to turn golden, remove the pan from the heat and pour the butter through a fine strainer lined with damp cheesecloth into a heatproof container. If the cheesecloth is damp, all the butterfat will pass through, otherwise some will be absorbed by the cloth. This method is a little fussier, but produces a clearer result.
Store clarified butter in the refrigerator for a few weeks or several months in the freezer.