Basically all the buildings are the same with different names for each age. But I'm gonna give you a sort of in-game perspective here. But before that you may want to know what determines how many buildings you can build per turn. This number is based on the total amount of land you have. So, as your amount of land increases, so does your number of buildings able to be built per turn also increase.


Past:

Markets: Gives your people a place to trade and do business. Which means more taxes, which means more money for you.

Huts: No not Jabba's but the little places where all your peasants live. Useful for getting more peasants and such.

Blacksmith: The place where all the weapons of your forces and parts for catapults and metal parts for your dragons are made, without these you have no new forces.

Keeps: Gets your troops and equipment out of the weather. This in turn makes your costs go down because you don't have to replace your forces materials as fast or feed them as much (its a well known fact you don't get as hungry when your sheltered).

Mage Towers: You guessed it a place for your mages to work and study their magic spells.

Farms: A place to grow grain to feed your peasants.

Towers: Give your empire a little more defense to prevent your enemies from breaking in.



Present:

Commercial Zones: A place for your people to work and make money and give you taxes.

Apartments: Efficient places for your people to live. A little crowded but, hey, your the leader right?

Industry: The place where your people feverishly work to build the war machines needed to help you win wars.

Bases: The place where your forces stay when their not fighting. More bases means less crowding, and they are all taken better care of with better training. Basically your expenses and prices are less.

Psi Centers: Places where your crack squad of telepaths goes to train and learn their art to protect you from enemy forces. It's these brave souls who do their best to stop the evil wizards, Master AI's and other disobedient present age countries.

Plantations: Basically big farms where you grow your food for your people.

Bunkers: Give your empire a little more defense to prevent your enemies from breaking in.



Future:

Replomats: Places where drones go to do their work. Items are made in such mass quantities that it's more like they're being replicated than made, thus the name replomat.

Giliad: Giant housing cubicles where drones come at night to regenerate.

Automated Factories: Factories completely run by machines. The drones are not intelligent enough or trusted enough to make the deadly war machines. And the AI's control is complete enough for it not to matter.

Storage Bays: Place where all your forces are stored when not in use. Again being stored cuts down on cost of maintenance.

Computer Centers: The heart and soul of your Master AI networks. Extending their network with more building makes them more able to control the wizards and telepaths that threaten you and yor people.

Hydrophonics: Buildings for mass food growth that are then broken down into basic easily consumable nutrients. Recycled humans help make the food especially nurishing.

Laser Cannons: Give your empire a little more defense to prevent your enemies from breaking in.