Is Warhammer Online instanced like Guild Wars?

I read in one review that Warhammer Online is instanced game like Guild Wars, but I do not know what that means. Can someone elaborate what “instanced game” means? From that review, I found out many good things about the game, and I am considering playing it. I like how PvP, PvE and RvR are played. Those elements are only working for RPG games and I like that fact.

As I understand, only the RvR battles and some scenarios are instanced. Everything else is just plain ordinary battles and quests. Warhammer Online is very addictive game, and already has millions of active players. I am one of them, and I am enjoying every minute spend playing it. RvR deciding battles takes places on objective-based battlefields and in instanced scenarios. This way, NPC merchants are one of the key figures in every battle. I recommend you to play the game. You will not be disappointed. Instanced can be fun and sometimes you will never see that a quest is instanced. It is great way to cover that in the game, because you will then enjoy playing the game.

not all rvr will be instanced, most will be open world play like in daoc, the instanced rvr they're talking about are scenarios like the WoW bgs. there will be instanced pve as well from what I understand, but again, that's not the entirety of it.