It has been a while since Blizzard gave the free realm transfer service to World of Warcraft players, but they are giving the offer now.
The offer lasts until Thursday, March 21 so if you want to make the switch be sure to act fast. Another reason to act fast is that the realm transfer service can be denied and the transfers for a certain realm can be closed due to so may anomalies involved in giving away such a service to such a large population.
Blizzard does this occasionally to benefit both, itself and the players. The dispersion of population should, in theory, decrease burden on a select few and reduce queue and wait time for the realms. The realms offered could be old ones with low population or new ones as Blizzards sees fit.
Guilds can be transferred indirectly because all the member will have to coordinate a mass transfer to a new realm and form the guild there.
There are some requirements to the transfer as well, you cannot be the leader of a guild, your mail and auctions should be cleared and you are advised not to log in until the process is finished. Read more about it on the Battle.net blog and also check out the FAQ section for more information.