Every five levels you guild gains a new raid. General concensus is that the raids are not for loot or even for lore. They are just for fun. You will find that every raid is incredibly easy and mentoring will nearly always be the order of the day. To add insult to injury just zoning into the zone gets the quest ding that you have completed the raid. All of the guild raid quest are given out by Royal Accountant Fowler in the Qeynos Harbor Scribe shop. In fact he will give you all of the ones your guild can do at once (ie. Level 5-20 if you guild is level 20).

The Avatars of Jealousy - Level 5 Guild Raid
Maximum Level: 23
Zone In: Eastern Antonica, The Shattered Vale
Named Mobs:

Sentry Gnolls (17^^ Group X2)
The Fang of Deceit (18^^ Group X2)
The Claw of Vengeance (18^^ Group X2)
High Shaman Vascha (19^^ Group X2)

Loot: Tier Two rare drops (Roughspun, Blackened Iron)

The Daughters of Lucan - Level 10 Guild Raid
Maximum Level: 28
Zone In: The Commonlands, Zarvonn's Tower
Named Mobs:

2 door guards (22^^ Group X2)
The Daughters (23^^ Group X2)
The Mother (24^^ Group X2)

Loot: Tier Three rare drops (Ruckas, Steel)

The One Who Got Away - Level 15 Guild Raid
Maximum Level: 33
Zone In: Antonica, Firemyst Gully
Named Mobs:

Piali the Profane (29 group x3)
Prince Mianja (28 group x2)
Envels the Heathen (27 group x2)
Shenot the Forsaken (27 group x2)
Taegso the Wretched (27 group x2)

Note: There are LOTS small groups of spiders, wasps, scorpions, etc. (27 group x2) that you might want to care in order to attack the named.

Loots: Tier Three rare drops (Ruckas, Steel)

The Scales of Justice - Level 20 Guild Raid
Maximum Level: 38
Zone In: Commonlands, Bloodskull Valley
Named Mobs:

Zealot Vukor (32 group X2)
Chief Surveyor Remolus (33 group X2)
Legionnaire Kruk (32 group X3)
Commander Gorzok (34 group X3)

Note: Once again there are small groups of elephants, dogs, rhinos, etc. (level 31-32 group X2)

Loot: Tier Four rare drops (Etched Leather, Feysteel)