Dark Eldar are physically like their uncorrupted counterparts, but chaos corruption has had massive consequences. Psychologically, Dark Eldar are notable for extreme sadism and aggression. Dark Eldar are also notable for their need to consume the souls of other life-forms in order to avoid being consumed themselves by Slaanesh. These two attributes drive the Dark Eldar to launch constant raids against inhabited worlds for slaves, which they use as playthings for their sadistic abuse and consumption.
The Dark Eldar are based in Commorragh, a massive city hidden somewhere in the Webway (although they do have minor presences outside in the physical universe) mainly to limit the total effect Slaanesh has upon them but also to defend themselves from retaliation. Dark Eldar society is cruel and harsh, being organized into various groups known as Kabals engaged in constant in fighting for position, while individuals also fight amongst themselves for positions both covertly and overtly. The position of a Kabal is based primarily upon the acquisition of slaves: the Kabals that acquire the most can use them as a type of currency to buy power. Various independent subgroups exist within Dark Eldar society -- including Mandrakes, Incubii and Wyches -- but they generally ally themselves with one of the Kabals.
The Dark Eldar are considered a threat to any civilization they come across, including other Chaos-based civilizations. They frequently make raids upon other civilizations, regardless of what faction they support. As they operate from the Webway, they are next to impossible to attack and are universally despised. Dark Eldar forces are highly mobile, have formidable firepower, and are fearsome in close quarters, but they are fragile and rely on hit and fade tactics in battle. Never the less, their presence in the Warhammer 40,000 universe is rather minimal due to their small numbers, and to the Imperium they are considered a nuisance compared to more pressing matters such as Chaos or the Orks. On the same note, the Dark Eldar (at least at present) have basically no interest in conquest, prefering to simply raid the greater galaxy from their unreachable city.