Raphyy,
I must say congrats as well. Hopefully it is the beginning of a lot more diving for you. If you are talking about the PADI Advanced Open Water cert. the differance is that they teach you more advanced skills (buoyancy, navigation, etc.) they also give you a taste of some of the specialties (wreck, deep, night, Dry suit, etc). What that means to you... the AOW will let you dive deeper, 130 feet rather than the 60 that you are limited to now. Also, you will get a bit less babysitting from any divemasters that you dive with. Basicly, this is the next step as you move from a new diver and want to expand what you can do safely in the water.
Rich