The Cylon data port is a device used by Cylons to access information in their datastream, as well as control various other Cylon technologies. Cylon agents need only to place their palm on the port and they are able to instantly access the data stream (Exodus, Part I).
Caprica-Valerii attempts a rough version of this technique with Colonial technology by inserting a fiber optic cable into her arm, thus allowing her connectivity to Galactica's computer mainframe (Flight of the Phoenix).
Data ports are also used on basestars in the ship's command and control center for data access and other ship functions. These data ports have a layer of water over the interface, which may have an aesthetic or functional purpose. These data ports (and the database circuitry used throughout the ship) are apparently organic in design, as they are damaged after an infection kills or shuts down almost every Cylon entity and device on the basestar (A Measure of Salvation).