Guilds are organisations of players working collectively in The Elder Scrolls Online who usually have similar interests and goals. When a guild takes control of a keep or one of its resources, the guild's store and bank are able to be used there via the keep's quartermaster. Only the store is available to the public, however.
Players can join a maximum of 5 guilds per account.
If an account makes a character from a different faction they can still chat in the guild and group up with players from the same faction as their character, but players will not receive the bonuses for any keeps or resources held if they are from a different faction than the guild's starting faction. 
Guild Stores work in two distinct ways. First, it will allow players in the Guild to sell items to one another internally. Secondly, if the Guild has captured a Keep/Resource in Cyrodiil, there will be an opportunity to sell to guildmates in that keep without having to teleport out of Cyrodiil.
In Guild Stores, players can set their own prices and sell any item they want, provided it isn't bound to their character or an Imperial Edition item.
Since there is no global auction house system in The Elder Scrolls Online, guilds can be created with massive amounts of players for the sole purpose of trading/buying/selling.
Guild Tabards are able to be purchased from the Guild Store, regardless of whether or not the guild has more than 50 members, but they will only appear blank until a Guild Heraldry is created (Available to guilds with over 10 members)
Guild Banks are storage areas for the Guild's members, and are unlocked once a guild reaches 10 members. Players can deposit or withdraw items and gold from this bank and the Guildmaster or an Officer is able to control what ranks are able to deposit and/or withdraw from the bank. Anything can be deposited in the guild bank, regardless if it is bound to your character or not and any item can be taken by any rank-appropriate player. Thus, it is not recommended to deposit your valuable items in the bank.
Guild Banks have a storage capacity of 500 items. There is no limit on how much gold can be deposited.
Guild Heraldry Edit
Guild Heraldry is unlocked once a guild has reached 10 members, and allows a guild to distinguish itself from other guilds. Only the Guildmaster is able to create and modify the Guild's Heraldry.
The Guildmaster first must purchase a tabard from the Guild Store, which costs 2000 gold. Then in the new Heraldry tab in the Guild menu, the Guildmaster is able to create the guild's very own tabard. They can select from a very large array of over 250 colors, 63 background styles and 136 crests, as well as a variety of tabard shapes and borders.
Guild Traders Edit
Guild Traders, also known as Guild Kiosks, are essentially public Guild Stores. Once a Guild wins the bid on, or buys out, a guild trader, the Guild's Store is able to be used by the public through that specific trader for one week (Or in the case of buyout, the remaining time left on the bidding cycle). Gold from sales is split between the selling player and the Guild bank. To use a Guild Trader, obviously, a Guild must have unlocked the Guild Store at 50 members.
Once the remaining time is up, the Guild loses the ability to sell through the Guild Trader and must bid on/buy it again.
Guild Traders can be found in every town in every zone with the exception of Cyrodiil, where Guild Stores can be used by the public through a specific keep if it is controlled by a guild, which does not cost any gold, as well as wandering the road.
Guilds start out with four ranks by default. A Guild can have up to 10 ranks which can be reordered, renamed and have their own logos. Any Guild can have any number of ranks regardless of member count.
There cannot be two Guildmasters, but a rank can be set so it has all permissions enabled (except for editing permissions, which is exclusive to the Guildmaster.).