The six playable races are:
Terran (free)
Aspha Miner (free)
Collective (free)
Marauder (pay to play)
Viral 106 (pay to play)
Guardian (pay to play)
The Manual lists the Mission requirements needed to unlock the race.
There are three other races that are Non-Playable, and are reserved for Special Events and/or staff member use to provide NPC style events at rare, random times.
The Non-Playable races are:
Dark Marauder
Kal-Zul
Berzerkers
There are no plans to make these races playable at this time.