Guild Banks is a feature added by Blizzard in Patch 2.3 to reduce the need for unofficial guild banks.
Access to the guild bank is made via 'Guild Vaults' which have been placed in near all bank NPC's. An advanced permission system is used to restrict or allow access to guild members and different bank tabs. In total, 588 slots are available seperated into 6 tabs, the cost of buying each tab increases rapidly, starting at 100g, then 250, 500, 1000, 2500 and finally 5000 gold. The guild bank also allows the storage of gold which can be withdrawn or used for Repair Equipment (subject to guild bank permissions).
Guild bank contents can now be view in The Armory once logged in.