Primary Statistics show how talented the character is in certain abilities. In the SPECIAL character system, there are 7 Primary Stats. Their first letter form the name of the system itself:
For humans, Primary Statistics cannot go below 1 or above 10 during creation. For other races (available only in Fallout Tactics), Primary Stats can go as high as 16. Each race or species has different minimum and maximum stat requirements. In Fallout and Fallout 2, the Primary Statistics cannot go below 1 or above 10 even with bonuses in game. In Fallout Tactics, bonuses from items can increase the statistics beyond their natural racial maximum.
Each race has some points distributed already at character creation. It does not, however, really matter, as they can be subtracted and added to anyway. Generally, humans have 40 Character Points to distribute among their characteristics. The numbers for other races are 42 for ghouls, 40 for super mutants, 41 for deathclaws, 45 for humanoid robots, and only 35 for dogs. Races with more than 40 character points generally have slower Perk Rate.
The Primary Statistics define the base stats for all Derived Statistics. See fitting Category for more information.