Guild officers are guild members who have been promoted into a role above standard members. Officers are authority figures in the guild, functioning as the leader's lieutenants and given many of the leader's most commonly used powers.
The criteria for promoting officers varies from guild to guild; some favor helpful members, those who are in game frequently, or those who show recruiting ability. Large guilds who recruit many unknown players may have a small core leadership team. Small guilds of trusted friends and guilds that are formed primarily as PvP teams often have most or all of their members at officer rank.
The exact intent of the officer position and what duties they are expected to carry out depend on the guild and it's leadership, but all officers in all guilds have the capability to carry out various functions, to: