Final Fantasy XIV World Transfer Service Is Coming Soon
You can transfer up to eight characters at once if it's a single destination. You cannot belong to a Free Company and you cannot have items being on sale on the market boards. The amount of gil you can bring into the destination World is limited.
The producer and director of FFXIV: ARR Naoki Yoshida also made an outline of the World Transfer Service describing an outline of the World Transfer Service describing what the service is, the benefits, risks, and case by case scenarios. Check it out here if you plan to transfer a character.