Starbases are large artificial structures, commonly used as a base of operations for either military or civilian purposes. Often, Starbases can only be moved via FTL techniques and can only either adjust their orbit or completely remove themselves from a harmful situation through faster than light travel.
It is very common for starbases to be modular structures capable of growing.
Military in function, star fortresses are characterized as staging grounds with large primary weapons for long-ranged attack and defense and thick armor. They are essentially orbital cannons with a carrier like functionality for the manufacture, repair and maintenance of orbital networks.
Artificial objects in orbit are maintained in position similar to a satellite network. Objects are often exchanged using linear launching systems, wire connection, cable transfer (akin to monorail). These are often living spaces, doubling as either mining operations or independent trading colonies.
A giga-cluster is a cluster of orbital network assets with a population greater than one million. Commonly, they are bonded into large crystalline honeycomb structures with clear roofs that from orbit resemble continents or tectonic plates and are often very large and very flat, perhaps only several miles thick but hundreds of miles across.
The interior of a gigacluster is often similar to that of a planet, complete with plants, soil, sunrise and sunset.