I just want to point out that the two other responses are slightly wrong - Caesar didn't add two months, and no months moved. There were always 12 months, and September was always the month with the fall equinox. What changed was two things:

1. The two months before september were renamed after Caesar and Augustus (I believe they had numbered names like the following months).
2. New year's was moved from March to January, thereby offsetting all the numbers by two from how they originally were.