Federation Space Systems was a shipbuilding entity that built Starfleet vessels for the United Federation of Planets.
The Constellation-class USS Stargazer was build by Federation Space Systems. (TNG: "The Battle", dedication plaque)
The Galaxy-class USS Enterprise-D was built by the Federation Space Systems Division at the Utopia Planitia Fleet Yards in the mid-24th century. (TNG: "Encounter at Farpoint", dedication plaque)
The Federation Space Systems was mentioned on the first iteration of the
Enterprise-D dedication plaque. 
According to Michael Okuda
, the FSS was also mentioned on the Stargazer dedication plaque. (X)