In a normal year, there are 365 days and 28 days in February. In a leap year there are 366 days and we add an extra day in February (February 29). Here you can see the upcoming leap years and what day the "leap day" is.
The earth uses approx. 365.2422 days in one round around the sun. In order for this to fit in with our calendar, we have inserted a leap year.
A normal year has 365 days. 28 of the days are in February. In a leap year, February has an extra day, for a total of 29 days. In a leap year, there are therefore 366 days in a year.
The reason the extra day was added to February, and the reason February is so much shorter than the other months, is that in the older Roman calendar, the month of March was the first month of the year and February the last.