Age of Conan's launcher indicates that the U.S. servers are offline for up to 24 hours while the American PvP shards (Cimmeria and Tyranny) are consolidated. Tomorrow will see the European servers undergo a similar process, with Crom, Hyrkania, and Ishtar merging into a single PvE mashup and Fury, Aquilonia, and Stygia joining forces as the remaining PvP shard.
Funcom has thrown up a server merge FAQ for your reading pleasure during the downtime, and in it you'll find details on everything from duplicate character names to duplicate guild names to the fate of guild-controlled battlekeeps. "Since build spots for battlekeeps are limited on each server, all build spots will be cleared and guilds will be reimbursed the resources for their battlekeep. This is to make sure that the guild gets the needed resources again and every guild gets the same chance," says the FAQ.
Read up on all the details at the official AoC forums.