The Ruins of Ahn'Quiraj, the 20-man side of the Ahn'Quiraj Complex. This is a rather easy dungeon, especially when compared to it's counter-part, the Temple of Ahn'Quiraj. Useful for testing out new players and getting a few side gear items, this is generally a breeze to complete.
A few of the bosses may pose some trouble but still, overall the dungeon is a bit higher then Zul'Gurub difficulty and lower in some cases. Do not, i repeat do not, use this as a gauge for whether you are ready for Ahn'Quiraj 40-man. The 20-man section is nothing compared to the 40-man version of Ahn'Quiraj
From the World of Warcraft Site:
During the final hours of the War of the Shifting Sands , the combined forces of the night elves and the four dragonflights drove the battle to the very heart of the qiraji empire, to the fortress city of Ahn'Qiraj. Yet at the city gates, the armies of Kalimdor encountered a concentration of silithid war drones more massive than any they had encountered before.
Ultimately the silithid and their qiraji masters were not defeated, but merely imprisoned inside a magical barrier, and the war left the cursed city in ruins. A thousand years have passed since that day, but the qiraji forces have not been idle.
Ossirian The Unscarred