The simple answer would the the Gulf current in the Atlantic ocean. Even though most of Europe is pretty even with Canada in terms of latitude, the warm water that travels from the gulf of Mexico has a warming effect. This also applies to air currents to some affect. It's why a city like London, which is far north of most Canadian cities, can maintain a relatively temperate climate as opposed to a continental city such as Montreal, which has a very high difference in temperature between summer and winter. A lot of the coastal cities in Europe (Atlantic coast) have a maritime or stable climate. But if you go inland towards Russia or even the plains of central Hungary, you'll encounter a more continental climate with more temperate extremes. Hope that helps.