What is the difference between "European Butter" and American butter?
Most butter in the US is sweet cream butter, meaning that the cream is not cultured before it is churned. European style butter is cultured by adding bacteria to the cream and giving it time to thicken and acidify before churning. This results in subtle flavor changes. European style butter also has a slightly higher fat content. 

FWIW, imported and domestic cultured butter is becoming pretty common on US shelves. Expect to pay considerably more for it.