I know this:
05-1.0 - highsec
0.1-0.4 - lowsec
0.0 or less - nullsec
If I understand it than:
highsec = only npc can attack you (in the asteroid belts - the only PvE place in the game).
lowsec = same as highsec but with extra PvP in the asteroid belts.
nullsec = only PvP.
And also as the number is lower then the npc are stronger.
Am I right?