Types of Butter

Salted Butter
The most common kind of butter, made from cream, and containing about 2% salt.
Reduced and Low Salt Butters/Sweet Butter
"Reduced salt" or "low-salt" butter usually has about 1% salt, half as much as standard butter. Unsalted butter has none, of course, but it doesn't keep as long; salt increases the shelf life as well as changing the taste. The name "sweet butter" usually means unsalted butter, though occasionally the phrase "sweet butter" is used for the majority of butters that are made with ordinary cream rather than sour cream like some of the cultured butters below.
Cultured Butter
Cultured, sour cream, or Danish-style butter has some selected culture is added to the cream and a different flavor develops, which is more acidic.
Dairy Blends
Dairy blends are a mix of butter and vegetable oil (up to 50%). They taste like butter, but have less milkfat, obviously, and are easier to spread after refrigeration.
Ghee, or clarified butter, is essentially just the milkfat and not the solids from the butter. It is often used for frying.
Whey butter
Whey butter is made from cream that has been separated from milk whey. This type of cream is left over from cheese making, which uses the curds of the milk and squeezes out the whey.
The use of butter is a cultural thing; for example, Northern Europeans and their descendants around the world use butter where Southern Europeans would use olive oil. Butter was forbidden on fast days and during Lent for Catholics, and one of Martin Luther's complaints against the Roman Catholic Church was the choice they required between importing olive oils from Italy with their attendant taxes, or buying indulgences allowing people to eat butter. He felt this was the Church's way of gaining revenue from Northern Europe. In 1520, he wrote that "Eating butter, they say, is a greater sin than to lie, blaspheme, or indulge in impurity." This everyday issue may have helped Protestantism catch on in countries where butter was a common part of the diet.

Carlson, Laurie Wynn. Cattle: An Informal Social History. Chicago: Ivan R. Dee, 2001.