We say that a year is how long it takes the earth to go around the sun. A day is the time it takes for the Earth to spin around so the Sun goes from being straight overhead (or at least not to the East or West) to straight overhead again. (One day is actually a little longer than the time it takes the Earth to spin once on its axis, because the yearly rotation also changes the Sunís apparent position in the sky.) However, the earth doesn't take an even number of days to go around the sun. The normal calendar year is 365 days. But the earth, like you said, takes about 365.25 days to go around the sun.
If we tried to add a quarter day each year, our times would become messed up and noon could come when itís dark at night, not when the Sun is overhead. Likewise, if we always ignored that 1/4 day, it would eventually be summer when our calendars said it should be winte.
So what happens is that every 4 years, they add an extra day at the end of February to make up for the 4 quarters of days that we ignored. That year is called a leap year.
It goes a bit further. The earth doesnít take exactly 365.25 days to go around the sun. There is a couple minutes worth of difference. Every hundred years (if the year is not divisable by 400), you have to skip adding that extra day in order to make up for the few minutes that we ignore each year.
(republished on 07/19/06)