On March 27, 2012, Final Fantasy XIV will see its first world merge. The goal of the merge is to enhance player experience during the period before FFXIV 2.0. New worlds may be added in the future, should the relaunch or general population see need of it.
Players will be able to select which world they wish to transfer to between March 1st and March 19th, and on March 27th the merge will take place. There is no timeframe for when the merge maintenance will take place.
The current 18 worlds will be merged into 10. Players will, again, be able to freely choose the world they wish to play on. For French and German players, while they can select any world, Square Enix is suggesting the choose New World 10 so as to make forming parties and such easier.
All the details regarding the merge, what will or will not be deleted, and other various things can be found on the Lodestone.