Mines allow your warband's economy to grow. These buildings will help increase your Per Capita Income, as well as producing an amount of cash themselves. New players should start out using mines as their main source of gaining money.
While workers will live on unused land, Tents are specifically designed for housing. As a result, they allow you to house a great deal more workers than otherwise. In general, at a low tax rate, markets will produce more cash. However, at higher tax rates (30-60%), tents will be more productive than markets. Remember that having a high tax rate lowers your maximum health.
Camps allow you to reduce your military expenses by more efficiently housing your units. They will also lower the price of all military units purchased from the black market. Those first two benefits do not increase once 30% of your total land are made up of Camps, so use a calculator if you need to so that you don't exceed this 30% limit if you don't want to. Camps also increase the rate at which your black market refills, so you might want to use them if you buy all of your troops off the bazaar and are running low.
Smithies produce your military units; the percentage of resources allocated to each unit type produced is controlled through warband management. They are one of the most commonly used structures because they allow you to make troops at lower costs than buying them. Make sure you go into the Manage page and change your Industry settings before building these.
Dens serve to train and house Spies, as well as produce Scrolls with which your Spies use to cast their missions. There are strategies in which you only build dens, but that might not be for you. Regardless, building a few dens is always a good idea. It allows you to Raise Defenses and Prepare Spies when needed.
Farms produce food, and are vital for feeding your workers and military. Without food, your people will starve and desert your warband. The more farms you have, the more food you produce each turn.
Towers or Guard Towers are strictly defensive buildings, worth 500 defense points each. Build these if you are very defensive and want to keep out all attackers. Once you pay the money to build them there is no upkeep to pay like with troops, but don't waste the money on them if you think that you will be broken anyway.
Your buildings or structures are what allow your warband to grow. Without them, you can't do anything. To build structures, you first need unused land (land with no structures built on it). You can get this land through attacking, scouting, demolishing, or hawk scouting. Once you have unused land, you have the option to build any of the structures above.
Located under the toolbar on the Build page there is some information that you should know before constructing. It shows how much each structure costs to build, the number of structures that you can build each turn, and the number of structures you can build total (with your turns, money, and unused land). Remember that each structure takes up one acre of free land. You can use this information in many ways.
Example 1: You have 10,000 acres of unused land, 80% health, and can build 250 structures a turn. You want to build as many structures as possible but not waste turns in which you won't heal. So if you do the math, you should build only 5000 structures. This will help you to maximize your attacks. Numbers normally won't be as even, but you'll get better at it the more you do it.
Example 2: If it says you can only build 5,000 structures even though you have 10,000 acres of unused land, make sure that your income is positive before building, or you will spend all of the money making the structures and lose your troops and workers because you have to money to support them.
The main thing that you'll notice on the build page is the table where you can type in the number of each structure that you want to build. There is also a box that you can click to build the "Max" or maximum number of the structure possible.
After you insert the values and want to build, just click the "Begin Construction" button.
The Demolish page, accessed from the toolbar on the left of the screen or from a link on the Build page is where you can demolish buildings and make free land. This is a useful feature that has several purposes. If you make a mistake when constructing, you can correct it by demolishing the right amount of certain buildings. If you are losing money, you could use it to demolish barracks, stopping production of costly troops and allowing you to build more markets. You also make a small amount of money for each building that you demolish (though it costs much more to construct and is therefore a poor source of income). You can also click the "Safe Demolish" box to demolish all of the tents and huts possible without losing any workers or hawks. Demolishing is also used in some strategies that completely change buildings at the end of a run. You could change all of your buildings into Guards to increase your defense and change the guards back into their original structures once you have more turns.
Make sure you know what structures your strategy requires before you start attacking and getting land. Also check your race's Build bonus to help gauge how fast you will be able to build structures. As you play more and more, the better you'll get at building and demolishing. You'll soon develop the ability to build at the right time between attacks and everything will be automatic. Good luck, and feel free to work your construction rats to death!