High sec = High security space (Value between 0.5 to 1.0)
Low sec = Low security space (Value between 0.1 to 0.4)
Null sec = Null security space (Value between -1.0 to 0.0)
* To check the value, look at
EVE Online: FA - Player vs NPC Corps. Pause at 0:25 to look at the top left corner: "Duripant 1.0". This is a high sec system with 1.0 security value.
Just like in the real world, high sec is a place with high police presence (CONCORD NPC police). You can still be rob/raped/bullied in high sec (if you are doing something that attracts attention like flaunting your "richness") but the police will apprehend the offenders.
Low sec has no presence of police, so it's kind of like a "wild wild west" where almost anything goes, but any illegal offenses will still be tracked by the police. Do enough bad stuffs and you'll have a hard time entering high sec space.
Null sec has no presence of NPC police too. However, the players are the one doing the policing job here to protect their respective territories. These areas are the "true wild west" where NPC securities do not give a shit what happens to you here. As mentioned before, these areas are mostly owned by players. You either sneak in or join the corporations/alliances owning those spaces.
Some terms definition:
High sec corp: Corporation that operates mainly in high sec space
Null sec corp: Corporation that operates mainly in null sec space
Corporation (EVE Online) = Guild/Clan (Most other MMO)
Fleet (EVE Online) = Party (Most other MMO)
Players + Players = Corporation
Corporations + Corporations = Alliance
Political Agreement between Alliances = Coalition/Bloc
This post has been edited by sumjs_5624: Feb 14 2015, 08:29 PM