These are different from normal banks in that they are communal. They are unlocked once 10 or more people have joined that particular guild.
The Guild master of the guild decides upon the access of the guild bank, meaning that certain ranks may or may not have access to the guild bank. With The Elder Scribes (this wiki's guild), for instance, access to the bank is restricted to the second level of guild membership, or those who actively contribute to the wiki.
Similar to other banks, they allow for a number of items to be stored in the bank. The total storage space in a Guild Bank is 500. Usually anyone can add to a bank, and place items into it. The idea of this is that it allows for more storage space for guild members for items that they cannot use, but want to share with other members. Good examples of these are raw materials — which cannot be sold for gold, but may be useful to other members.