Many baking recipes call for "softened" butter, or roomtemperature butter. The reason for using softened butter is that you want to beatthe butter (also called "creaming the butter"), either with sugar or without, and it's very hard to beat a cold stick of butter.
The best thing to do to soften your butter is to put it out on the kitchencounter for an hour before you have to use it. But sometimes, we don't think that far ahead. One popular solution is to put the cold butter in the microwave for 10 to20 seconds. The problem with microwaving, butter is that the microwave can heat the, butter unevenly, and tends to over-soften or even melt the butter in places. You want pliable butter for beating, not almost melting butter.