A guild hall is like a shared apartment for the entire guild. Guilds can use them as places to meet or as shared storage space.
A guild can purchase a guild hall in any of the 4 capital cities (Pyr, Yrkanis, Fairhaven, Zora) by going to a guild apartment caretaker and purchasing it for 10,000,000 . Once purchased, all members of the guild can enter the guild hall through the door behind the guild apartment caretaker where the guild hall was purchased.
Each of the 4 capital cities has multiple guild apartment caretakers, and once the guild hall is purchased it can only be entered through the door behind the caretaker where it was purchased. So chose wisely the most convenient location before you make the purchase.
Any number of guild halls can share the same entrance. Because some locations are popular, often many guilds have their guild hall share the same entrance; any player entering the door will automatically be send to the correct guild hall.
The guild hall has it's own inventory space, with a maximum capacity of 10,000 bulk or a 1000 item slots. It also has a shared dapper deposit. Any guild member can put items and dapper into the guild hall's inventory, however only the guild leader and high officers can take items and dapper out.
You must be inside the guild hall to put anything in the guild hall's inventory or take anything out. The interior looks of the guild hall depend on which capital city it is located in.
Every guild hall also has 4 guild trainers, but these currently serve no function.
A guild can have only one guild hall at any time, in only one of the 4 capital cities. If a new guild hall is purchased while the guild already owns one, the previous guild hall and all items in it are lost. Only the guild leader and high officers can purchase a guild hall for the guild.
For more info on how guilds work, see Guild.