Butter comes in two forms: salted and unsalted. Salt is added as a flavoring and preservative agent. In addition, sometimes a coloring agent (Annatto) is added and salted butter contains higher water content. However, salt can overpower the sweet flavor and mask any odors. I prefer to use unsalted butter because of its fresher taste and because I have read that the amount of salt added varies therefore, it is hard to know how much extra salt to add to your recipe. My rule of thumb is that if I am substituting salted for unsalted butter in a recipe, I simply omit the extra salt in the recipe and then add more to taste if necessary. Please note that unsalted butter has a shorter shelf life because it contains no preservatives. Most butter has an expiration date on it so if you do not use butter frequently (is that possible?) or buy in bulk as I do, just freeze it. You can freeze butter for around six months but be sure to wrap it well for longer life and to prevent it from picking up odors. When you are ready, simply toss it into the refrigerator overnight to thaw.