The Lanteans, also known as the Ancestors in the Stargate Universe, were those Ancients who left Earth in the City-ship Atlantis 5-10 million years ago and settled in the Pegasus galaxy. They were directly responsible for the emergence of the Asurans and the Wraiths, and seeded Pegasus with Humans. Eventually, being Ancients, they ascended.
The Lanteans were worshipped as gods by the people of the Pegasus galaxy and some of their technology was left behind to be found and used by people who understood it. Atlantis was among the many wonders left behind and was ultimately discovered by an expedition; unknown to them Atlantis was the centerpiece of Lantean society and was a massive City-ship capable of space travel.