An acronym for Player Versus Player, PVP refers to players battling with other players. Heroes is the Ashen Empires Server which allows PVP.
While the majority of the Heroes is open-PVP, there are some exceptions in the form of localized Safe Zones, where players cannot be targeted at all, and No Drop Zones, where players may be targeted but do not drop their backpack or damage gear. Being Criminal, where you attack a member of the same faction or attack a character twenty levels above or below you, allows you to be targeted and killed in Safe Zones and No Drop Zones alike. This lasts for 10 minutes until you either regain lawful status or are temporarily sent to jail.
All players under level 106 may turn their PVP off on Heroes. This is removed for character levels 106-110.
A non-PVP server is a server without PVP. On Legends server, Player Versus Player is disabled, meaning that players cannot target other players with an attack outside of designated PVP zones, such as the Guild Arena or Lotor's Duel Arena, which are also common to Heroes server.
With the introduction of "Factions" in 2008, PVP rules became quite complicated and have since been revamped several times. Suggest reading the information on the AE website at Ashen Empires - Player vs. Player. Factions also exist on Legends but do not involve PVP. As of mid- to late-2011, the faction debate is ongoing, and it will likely come under close developer scrutiny for the next update.
Currently, there are three factions: Lotorians, Talazarians, and neutral.
Members of one faction may attack members of the other without going Criminal, or "temp" as it is referred to colloquially. Similarly, characters can target most NPCs of the opposite faction without losing alignment or gaining Criminal status. As such, a Lotorian entering a Talazarian town will be set upon by the town guard and killed (instead of being jailed, like a Criminal would be), and vice versa. Neutrals may enter all towns at will and suffer no criminal status for attacking either faction, NOR for attacking other neutrals. Members of one faction may resurrect, apply first aid to, and cast buffing spells only on members of their faction, though neutrals may resurrect members of any faction.
Neutrals may not claim cities for their faction, as Lotorians and Talazarians can. The benefit to claiming towns is a temporary buff (1 hour) granted to a member of the claiming faction upon speaking to the local Town Wizard of Insight. (Note that a few towns do NOT have these NPCs. This is developer oversight and will likely be fixed in the next update.)