The datastream is the central computer network inside a Cylon basestar.
The datastream is literally composed of water, which is used in whole or in part as a transmission medium, a storage medium, or both.
Several Cylon agents are seen using interfaces in a basestar's command and control center that consist of a illuminated panel covered by a thin layer of water, suggesting that the water may serve as both "display" and "input device" as an agent interfaces the datastream. This behavior is reinforced by the comparative lack of visual displays throughout the command and control center. All information appears to come from these watery data interfaces.
The interfaces are normally used by the Cylons by placing their hand on the interface, making contact with the illuminated base and the thin water layer, which activates the data ports within their arm. The interfaces can also be configured to show visual information such as still photography, such as the photos taken by Gaius Baltar during his exploration of a dying basestar that makes a reconnaissance mission to the Lion's Head Nebula (Torn).
The basestar's central computer, the Hybrid, rests within a pool of water, connected to many conduits below the water. Water drips on the Hybrid from above the pool, suggesting that data might be relayed to the Hybrid from the water surrounding and falling upon her in some fashion.
Athena accompanies fellow members of a Colonial boarding party that investigates a basestar that is found disabled in space. She instructs others to place "SSR leads" in a water pool as she attempts to access the basestar's computer database, hoping to make a data dump from the basestar to a Colonial data storage device. Athena finds the "datapoints" severely corroded, suggesting that the datastream has an organic element that is as equally susceptible to damage as the Cylon agents.
After living Cylon agents are found and information on the ship's infection is discovered, the boarding team appears to abort their information gathering attempt (A Measure of Salvation).
Further information on the datastream's network topology or network design is not revealed.