Unsalted butter tastes better!
Plugra or Keller's are European-style cultured butters. Land O'Lakes is
another brand you might find, not cultured. Try to buy these frozen. If
they're in a refrigerator in the store, ask if they have some in their
freezer in the back and buy that.
European butter also has a high percentage of butterfat, 82 percent to 86
percent, and so a lower percentage of moisture (so don't warm it in the
microwave, it could damage the unit).
The best way to learn the difference is to buy some and experience it yourself.
Cheerz,
--Larry