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Every character can be memeber of more then one guild, guildmemberships are account bound. You can change your current guild on the fly, you only get access to the guild that is active, there can be only one guild active. So you could have sepperate guilds for PvP, RP, PvE endgame WvWvW...
You only get access to the guildbenefits of your active guild, also you can only have access to the guildchat of that active guild.
For your active guild you can earn influence, The guild can spend that influence on things like flags that give XP bonusses to members, but also guildvaults and such. In WvWvW this guildinfluence can be used to take a keep and to strengthen its defenses.
Best MMO experiences : EQ(PvE), DAoC(PvP), WoW(total package) LOTRO (worldfeel) GW2 (Artstyle and animations and worlddesign) SWTOR (Story immersion) TSW (story) ESO (character advancement)