Why does the month of February have 28 days and a 29th day in the leap year?

There are a couple parts to this question.

First of all, February has one more day in a leap year because in a leap year one day must be added to the calendar.  This is because the Earth actually orbits the sun in roughly 365.25 days.  So every 4 years we need an extra day.  Otherwise, the dates would stop corresponding to the right seasons and we would have, for example, Christmas in the middle of Spring in the Northern Hemisphere.  So, to keep the calendar date matching with the seasons, we have to have leap years.

The second part of this question is why February has so few days.  My understanding is that it used to have 30 days.  But then Julius Caesar wanted another day for "his" month of July.  So he took one from February.  Augustus Caesar didn't want Julius's month to be longer than "his" month of August so he took another day from February.