Command Center - The Command Center is the center of every colony. Each colony must have one and may not have more than one per planet. Despite their somewhat grandiose appearance, all they really do for you is build Engineer units and provide housing for 10,000 Colonists. It provides the colony with no storage capacity whatsoever.
The starting Command Center is unique in that it comes with a radar with a maximum range of 255, allowing you to see flashing blips a considerable distance away. However, if you decide to design a new Command Center, its radar will only be as good as the one you install in it.
Power Generator - Power Generators are structures specifically designed to generate power. Any structure can be made to generate power, these were simply built specifically for the task.
Barracks - Barracks train a number of Colonists into Enlisted troops.
Officer's Training - Similar to the Barracks, the Officer's Training structure converts Enlisted troops into Officers.
Factory - Factories are capable of producing all ground vehicles and components. They also provide the colony with a good number of jobs.
Spaceport - Spaceports are capable of producing flying units up to the size of their largest bay door. The bay doors can be enlarged a great deal over the starting Spaceport, but eventually large ship construction will need to be handled by orbital space stations. Be careful not to build a ship that is too large or it will be trapped in the Spaceport.
Storage Facility - Storage Facilities are structures full of cargo space. They are primarily used to store minerals. Note that a Storage Facility can be built using any of the structure Hulls. It does not have to use a "Warehouse" Hull. However, only the Warehouse Hull and the Refinery Hull count towards a colony's shared storage capacity. You do not start out with any kind of Storage Facility, so this is usually one of the first structures players design.
Turret - Specialized combat structures. Unlike units, turrets can draw power from the colony's power grid. Like Tanks, Turrets are immune to critical hits. Turrets have a 1000% bonus to armor.
Refinery - Refineries are both a drop-off point for ore from the Mining Facilities and they are also used to produce any Alloys that you have created. Refinery cargo space counts towards a colony's shared storage capacity.
Mining Facility - Mining Facilities extract minerals from mineral deposits. Note that Mining Facilities are considered public facilities! This means that anyone can use anyone's Mining Facility.
Research Facility - The Research Facility is the only structure capable of conducting research. They are an essential part of any empire.
Residential Facility - These structures house your colonists. As such they will reduce Homelessness on your colony window if you have any.
Tradepost - Having a Tradepost allows your colony to trade with other players. You may only build one per planet.