Tomorrow, Wednesday, is Leap Day, Feb. 29 with that meaning 2012 is obviously a Leap Year. Leap years are those which have an extra day, used to keep the calendar year synchronized with the astronomical or seasonal year.
It's done because the year isn't really 365 days. Rather, 365 days are shorter than a solar year by nearly 6 hours.
Without this adjustment, we'd see the seasons eventually drift. Given that, in the Gregorian calendar, which is a modification of the earlier Julian calendar first used by the Romans and is the standard calendar used in most parts of the world, most - but not all - years that are evenly divisible by 4 are designated as leap years.
There are exceptions to this rule because, as noted above, a solar year is slightly less than 365.25 days. To further adjust for that fact, years that are evenly divisible by 100 are not leap years, unless they are also evenly divisible by 400, in which case they are leap years.
For example, 1600 and 2000 were leap years. However, 1700, 1800 and 1900 were not leaps years. In the future, 2100, 2200, 2300, 2500, 2600, 2700, 2900 and 3000 will not be leap years, but 2400 and 2800 will be leap years.
Technically, there will still be drift. The Gregorian calendar was designed to keep the vernal equinox on or close to March 21st, so that Easter remains correct with respect to the vernal equinox. That said, the vernal equinox year is about 365.242374 days long (and increasing). Using the above methodology, the Gregorian calendar year is 365.2425 days long.
Given this, in 8,000 years, the calendar will be about one day behind. Also, in 8,000 years, the length of the vernal equinox year will have changed by an amount that cannot be accurately predicted (as it is increasing) and thus this one day off may or may not be of consequence.
A person born on February 29 is sometimes called a "leapling" or a "leaper". In non-leap years they will generally celebrate their birthdays on February 28 or March 1 (with March 1 being somewhat more likely as technically it is the day following February 28).
Obviously, missing a "true" birthday in a non-leap year doesn't mean a person is younger in age. In reality the person isn't missing his birthday so much as having fewer birthday anniversaries than their age in years.
Image Source: Wikimedia Commons