On iPhone, the screen is divided into three columns.
The first column, at left contains your clan's basic resources: Herds, Goods, and Food Stockpile.
Herds represents your clan's livestock, a form of wealth that naturally increases and also produces food. You can trade your herds to other clans, but it's often not the best option. Your Cattle number is always the same as the Herds number at the bottom of the screen, and your Goats number is always twice that; Horses are calculated separately.
Goods are your trade goods. This is an abstraction of a wide variety of goods, including cloth, metalwork, and so on. Goods are measured in cows: 1 good is worth 1 cow.
Food is measured in seasons stockpiled. Your clan will constantly produce a bit of food, although the big harvest is in Earth Season. If you have a surplus, you can trade it; if you have a deficit, you can trade for food.
The central column deals with your clan's rarer resources
First is the list of your Treasures. Tap a treasure's name to get its description.
Underneath the treasures is a list of your Exotic Goods. Some, but not all exotic goods allow you to have an extra trade route.
The last column, at right, deals with your clan's current trading situation.
Depending on the number of trade routes you support, your Market can be annual, seasonal, or weekly. The more frequent the better: a weekly market generates more trade goods than a seasonal one, which in turn produces more than an annual market.
Your clan's Trading Partners are the clans you have trading routes to. The more you have, the more frequent your market will be, as the market represents how often your trading partners visit you to trade.
Finally, the Caravan button allows you to send out a caravan, and takes you to a new screen to manage that.
Again, on iPhone this screen is divided into three columns.
The first is a list of known clans, to which you can send caravans. Next to the "Send To" caption at top is a map icon (a quartered circle). Tap it to look at the map and check a clan's location before sending a caravan--if you have not yet performed Taming the River for the Black Eel or the Oslira, a caravan can be unable to cross the relevant river to reach its destination. In this case the caravan will have to return home without trading, wasting a turn.
Clans you have already sent caravans to are marked with a check; keep track of this, as sending a caravan to all clans of a given culture has benefits. Most tangibly, it allows you to support an additional trade route.
Clans marked with a N are your neighbors. Caravans sent to them will return much more quickly than those sent to other clans.
The central column describes the goal of the caravan you are sending out. You can send two types of caravan: a normal trading mission, or a mission to establish a trade route. The "establish trade route" option will be unavailable for clans that are already your trading partners.
If you take the other option, you must decide whether to send a small, normal, or large caravan--you can control the size of the caravan by tapping the dots next to Buy/Sell. The more clustered dots in the circle, the larger the caravan. You also have to decide what you want to buy or sell. The options are Food, Goods, Herds, Horses, and Treasures. Most of these can be combined however you like--you can sell Food to buy Goods and Herds, or sell Goods and Herds to buy Horses--but Treasures are an exception. If you choose to buy a treasure, you can only decide whether you want to buy a treasure, sell one, or trade treasures; you can decide what you want to offer for a new treasure once your trader reaches the other clan. And if you choose to sell a treasure, you will need to select which one from the list at the bottom of this column.
There are limits on what you can buy or sell. You cannot trade herds with the Pure Horse Clan, or horses with Rams. If you have a surplus of food, you will not be able to buy it; and if your food situation is bad, you will not be able to sell it.
Finally, the column at right deals with the clan members you are sending on this caravan. The leader must be one of your pool of nobles. The default choice is usually the best bargainer available; you send someone else if you prefer, for example if you want to keep a Clan Circle member home instead of sending them to trade with a faraway clan.
You also have to decide on the caravan escort, sending Swords and Bows to keep the trader and goods safe. Keep in mind that these fighters will be unavailable to fight in raids or defend the clan until the caravan returns! If bandits are a problem, you'll want to send a larger escort; otherwise, a smaller one is all right. Your Clan Circle will tell you whether or not bandits are a problem right now.
For more details on caravans, see Trade.