The Wraith of Stargate Atlantis are a race of psi-vampires that can give and take life force by pressing their hands against someone's chest, causing them to age rapidly if taking and grow younger if giving.
Evolved from the Iratus bug in the Pegasus Galaxy. Ten-thousand years ago, they went to war with the Ancients (who, at the time, were known as the Lanteans to them) over control of the galaxy. The Ancients became careless and sent ships deep into the Wraith's hives, but the Wraith caught three ships and were able to clone theselves, increasing their army one-hundred fold. Eventually, the Ancients submerged and went back to earth with Atlantis.
First encountered in the pilot episode "Rising", the Wraith capture a number of personnel while scouting Athos. The "Keeper" reads Sumner's mind and sees that Earth is flushing with life and energy. Sheppard kills the Keeper, but only to awaken the rest of the hibernating Wraiths. Hive ships are now all over the Galaxy, but in "Poisoning the well", the Hoffan drug is introduced and is proven to be poison against the Wraiths. At the end of the season, they discover the Wraith are after Atlantis.