I would just refer you to this page: Memory Alpha:Stardate, but let's be sporting about this.
1- A random set of numbers that usually increase as time gos on. Stardates in the The Original Series were for the most part random.
2- In The Next Generation, the first two numbers were the century, 24th, and then the season of the show. The remaining numbers were space out to be a full "year."
3- In the new movie, the stardate is the Gregorian year with the number of days as the number after the decimal. IE: 2340.366 would be the last day of the year in a leap year, or 2333.45 would be the 45th day of the year.
4- "In universe," things get more complicated, in which case click the link at the top. - Archduk3 03:39, 3 July 2009 (UTC)