This just in from the Official WoW forums - NOT written by me, please see this post for the original writer, Evol from Chupathingy:
Why Guild Banks?
Guild Banks were introduced in patch 2.3 in order to help reduce the need for player created banks, and also to reduce the temptation for "account sharing" which sometimes happened with such a system.
Where are the Guild Banks?
The Guild Banks are accessible through Guild Vaults, located in or near the NPC bankers already in use.
How does the Guild Bank work?
The Guild Leader has rights to set permissions for the Guild Bank in the Guild Information tab. Permissions are set by rank, just like other permissions.
The Guild Leader can set the maximum amount of gold able to be withdrawn per day, and can individually set whether a rank can view or deposit to each tab available, as well as how many items or stacks per day a player can withdraw.
How many tabs are there, how much do they hold, and how much do they cost?
There are six tabs available for purchase, each with 98 slots. There is no need for bags. The cost is 100g for the first tab, and the cost increases with each additional tab (250g, 500g, 1000g, 2500g, 5000g)
Can I use the bank for guild repairs?
Yes. Repair NPCs now have a 'pay for repairs from guild funds' button available if your Guild Leader has set up permissions for you to be able to withdraw funds from the bank. You can use this to repair up to your daily withdrawal limit, if you have the permission set as such.
Is there a way to keep track of transactions?
Yes. In game, each tab keeps a log of the last 25 transactions, viewable when you visit the Guild Vault. In addition, the armory (www.world of warcraftarmory.com) shows the contents of each tab, and logs the last 1000 transactions.
When I try to set permissions, it keeps setting everyone to Guild Leader. How do I fix it?
Try logging in with no mods enabled to set your Guild Bank permissions. There is a bug in some mods (we haven't narrowed down which yet) that doesn't play well with the Blizzard UI when setting ranks and permissions. You can turn them back on once you've set your permissions.
Can I name the tab something other than Tab 1?
Indeed. Right-click on the tab icon, and you can set the name and icon for the tab (much like you do when creating a macro).
Can I move items from one tab to the other?
If you have sufficient permissions, you can move items from one tab to another, and it shows as such in the bank log file.
Why can't I open my Guild Bank?
Assuming you have rights to view it, and are in the right place and clicking on the Guild Vault, some mods interfere with the Guild Bank UI. For example, if you are using dkpsystem.com, the older character profiler was named Guild Bank, which causes a conflict with the named Guild Bank in the game. In this case, a new mod was released with a different name (dkp profiler).
If you are having this problem, check to make sure none of your mods have Guild Bank in the name, and/or try disabling mods to locate the conflict or try turning all your mods off.
Do I need to use this Guild Bank?
Absolutely not. If you've already got a good banking system in place, or have never had a need for a guild bank, then there's no reason you have to use this one simply because it's in the game. However, if you do decide to use it, it is a convenient way to share items with your guild.
I don't want the Guild Bank used for repair fees. How do I turn it off?
Set the amount of gold available to be withdrawn for each rank to 0.
Is there a way to set someone else up to run the Guild Bank other than the Guild Leader?
Not directly, though you can set a rank for them that has all of the permissions. The Guild Leader will still have to set permissions for ranks as well as purchase tabs.
How do I give a rank unlimited withdrawals of items or gold?
You can't, technically. What you can do is set them up with a ridiculously high number of withdrawals (like 100000) for each tab plus gold withdrawals and it will achieve the same thing.