The Cytherians were an advanced species who lived near the center of the Milky Way Galaxy. They were explorers and made contact with various other races. The manner in which they initiated first contact was atypical; they brought other races to them rather than traveling the galaxy in search of intelligent life.
In 2367, the USS Enterprise-D encountered an alien probe while attempting to repair the Argus Array. The probe interfaced with Lieutenant Reginald Barclay, and gave him the advanced knowledge needed to bring the Enterprise-D through a subspace distortion to the Cytherians, the presumed makers of the device. After the Enterprise-D had arrived at the Cytherians' homeworld, Barclay was returned to normal. (TNG: "The Nth Degree")
The script notes for "The Nth Degree" described what appeared simply as "a holographic-type image of an alien being. It is an odd combination of intense curiosity, lightheartedness, quickness and benevolence."
According to an internal reference document, listing all the aliens seen in Star Trek: The Next Generation, which was circulated around the time of the production of TNG Season 5, the Cytherians were described as "elderly, bearded human-like men, with blue patch on forehead."
The Cytherians return in the Pocket TNG novel Takedown, where a renegade Cytherian named Proctor augments eight 'diplomats' from various key powers in the Alpha Quadrant and 'programs' them to attack assorted long-range Federation communication outposts; the intention is that, while the attacks themselves are not intended to cause any actual loss of life, the resulting hostility will lead to the Cytherians being identified as the cause of the attacks and encourage them to remain in isolation. However, the truth is exposed when Admiral William T. Riker is able to complete his 'assignments' faster than anticipated by transferring from his flagship, the USS Titan, to the faster USS Aventine, allowing him to delay his return to the Cytherian outpost and provide the USS Enterprise-E with the clues needed to work out what was going on. This results in Picard and Riker making contact with their original Cytherian contact Caster, exposing the deception, and helping the Cytherians establish clearer limits for their future interactions with the galaxy.