Salt is added to butter as a preservative and up until the 1970s, salted butter was the norm in many houses while unsalted butter was rare. My experience is that today it's the opposite and that unsalted butter is the more popular butter.
Most people feel that because salted butter can last much longer in the refrigerator than unsalted, it tends to be less fresh. They also like controlling the amount of salt in recipes and since the amount of salt in salted butter can vary from brand to brand, buying unsalted butter is the way to go.
Things can get further complicated when butter is labeled 'sweet cream' as sweet cream can be both salted and unsalted. Sweet Cream is usually used a way to differentiate it from cultured butter which has a richer, cheesier flavor. While most brands are making it easier to tell the difference between salted and unsalted by color coding their packaging, it still is a good idea to double check the label and be sure that you have what you're looking for.
(Image: Faith Durand)