The company that developed Descent and Descent ][ was called Parallax. This company later split into two seperate companies: Outrage, which created Descent 3, and Volition, which created Freespace, Red Faction, Summoner, and their sequels.
So Descent: Freespace is essentially saying 'An open-space combat sim from the creators of Descent.' There's lots in common between them, as far as coding and rendering methods go, and there's lots of similarities in the fancy explosions with lots of debris and innovative re-working of the same engine to create a sequel.