Why do we have an additional winter day every four years? What do we call a year containing 366 days?

We have an additional day every four years (29th February) because it takes the Earth 365 and a quarter days to move around the Sun. As we don't count an extra quarter of the day at the end of a year, we add an additional day every four years. These years are called leap years. A year containing 366 days is called a leap year.