Planet X was the homeworld of an unnamed and enterprising alien race conducting trade with a paralell universe version of Earth. The planet was the headquarters of the X.S. Tech corporation under the leadership of Chairman L.C. Clench. Representatives of Chairman Clench's species were first encountered by a stranded away team from the USS Voyager.
According to observations made by the Voyager away team members Planet X appeared to be a rather capitalistic society. The X.S. Tech corporation appeared to be an influential force in their society. It was believed, but never proven, that the pursuit of profit drove the inhabitants of Planet X to the stars.
The inhabitants of Planet X were green skinned and hairless humanoids hypothesized descended from a repto-mammalian ancestor according to William Telfer.
Their level of technology was assessed to be equal to the United Federation of Planets in most respects and more advanced in others. In the areas of robotics and cybernetics, Planet X was superior to the UFP creating such artificial constructs as Bishop, the android surgeon who treated an injured Celes Tal, the XMD-1 surgical robot, and the Simulated Intelligence Robotic or S.I.R. technology demonstration robot.
In the area of transporter technology the inhabitants of Planet X were on the verge of testing an interplanetary teleportation device, the demonstration of which the Voyager away team would witness. (Star Trek: Voyager - Dark Realm)