A guild zone is a piece of the world that belongs to one guild. The guild buys it, teleports to it whenever it likes, and nobody else can touch anything the staff places inside it.
You can always ask the stuff to customize your guild zone even further.

How a guild gets one

Talk to the Guild Zone Broker in any major city. What the Broker offers depends on who you are:

Only the Guild Master can buy or sell. Every member can teleport.

Preview before you buy

Every zone on the list can be previewed before spending anything. Preview teleports you to the zone for 150 seconds, then brings you back to exactly where you were, automatically. You can preview as many zones as you like, as many times as you like, for free.

The price

Zones are bought with Tokens, and each zone has its own price set when the staff creates it. Selling a zone back refunds 50% of what you paid.
A guild owns one zone at a time: the purchase list only appears while your guild owns none.

What “yours” actually means

Everything the staff places within 100 yards of the zone marker — vendors, trainers, portals, chests, decorations — can only be used by members of the guild that owns that zone.
Everyone else can walk in and look around, but nothing inside will respond to them. An unclaimed zone is closed to everybody until someone buys it.

Buying a zone now does more than before

Owning a zone is no longer only a place to stand. The moment your guild buys one it becomes a guild house, which unlocks the guild achievement [Homeof Our Own] and switches on a permanent set of bonuses for every member.