For many, clarified butter is the cooking fat of choice in the kitchen. It is butter where the milk solids, water and whey proteins have been removed. The resulting butter is a beautifully clear golden liquid when melted, preferred in many recipes because it can be cooked at higher temperatures than standard butter. (The milk solids in standard butter can easily burn.)
Because it's clarified, this butter can also last longer -- the milk solids that can cause standard butter to go rancid so quickly have been removed. Of course, the milk solids also impart rich flavor, so clarified butter will not have the same depth of flavor as standard butter.
You can find clarified butter (or ghee, a type of Indian clarified butter) in many cooking stores and specialty markets, but it can be costly, and it's just as easily made at home.
To make your own clarified butter, gently melt the butter over low heat in a heavy-bottom saucepan (or slowly melt it in the microwave). As the butter melts, the milk solids will sink to the bottom and water and whey proteins will gently bubble to the top. After the water has evaporated, gently skim the whey (the dry foam) from the top of the clarified butter. Very gently pour the clarified butter into a separate container, making sure not to disturb the milk solids at the bottom. 1 cup (2 sticks) of standard butter should give you 1/2 cup to 3/4 cup clarified butter.