The Ancients are a technologically and physically advanced alien species who have the ability and knowledge to harness wormhole-based technology.
The Ancients originally existed in another realm, one that is vastly different from our own. This plane of existence was bridged to ours by wormholes. When they became aware of the various species in our realm, and many of those species' aggressive tendencies, they modified several members of their own species, allowing them to live and exist in our universe. These individuals became the Ancients and they were tasked to catalogue, report, and influence the activities of the species in our universe.
After many years in our realm, however, the planet where the Ancients lived began to die. They began a search for a suitable location whose dominant species would welcome them and where the Ancients' hive could live and replenish. The Ancients only had enough power to transport their entire race one last time and because of their unique nature, they could only live on a small number of worlds. Discovering that Earth was one of these worlds, the Ancients create an elaborate simulation using the memories of the human John Crichton.
Creating what appears to be a wormhole leading to Earth, the Ancients lure John into this simulation. There, Crichton's memories and expectations play out a scenario that allows the Ancients to see how humans would respond to alien contact. Shortly after Crichton realizes that there is something very wrong about the Earth to which he's returned, an Ancient in the guise of John's father Jack Crichton explains the situation and the reason for the simulation. Having learned that humanity would not welcome an alien species on its planet, they continue their search for a new home. Unknown to Crichton at the time, "Jack" gives him subconscious knowledge of wormhole navigation. His intention is to provide Crichton with a potential way home if he proves to be worthy by unlocking that information himself. The Ancients eventually go on to discover a planet that is suitable for their needs.
On their way to this new planet, the Ancients discover what appears to be Crichton's Farscape-1 module being piloted into an unstable wormhole by a Charrid. Believing that Crichton misused the knowledge given to him, "Jack" seeks him out. He finds Crichton traveling on Talyn and he confronts him. They soon realize, however, that Crichton had shared what limited wormhole knowledge he had before he had met the Ancients--to a mechanic named Furlow, on the planet Dam-Ba-Da. Traveling there, they find that Furlow has built a replica of John's module and is using it to unlock the secrets of wormholes so she can sell it to the Scarrans. Though "Jack" is eventually killed (by Furlow) in the encounter, he does unlock the wormhole knowledge in Crichton's brain and they construct a displacement engine, a device that alters a wormhole to gather a large amount of plasma from a nearby star to be used as a weapon. They then use it to destroy a Scarran dreadnought in order to keep wormhole technology out of their hands.
One cycle later, the other Crichton encounters a being who belongs to the Ancients' species, though he has not been modified to exist in our realm. This individual, nicknamed "Einstein" by Crichton, is curious about why the Ancients decided to give Crichton wormhole knowledge. Einstein explains that they had originally investigated why his species has disappeared from our universe. In doing so, they were alarmed to discover Crichton's extensive knowledge of wormholes and the many aggressive species hunting him as a result. Though Einstein originally intends to end the danger by killing Crichton, he eventually lets John go, albeit after warning him about the many dangers of careless wormhole travel.
Months later, Crichton takes Scarran Emperor Staleek to see Einstein to confirm for him that Crichton cannot make a wormhole weapon. Einstein later gives Crichton the knowledge needed to construct such a weapon. After Crichton uses the weapon to intimidate the Scarrans and Peacekeepers into declaring a truce and ending their Peacekeeper/Scarran War, Einstein removes the all wormhole knowledge from Crichton's brain. With the knowledge gone, Einstein's original mission is complete and he presumably returns to his own realm.
In their true form, the Ancients are thin and bipedal with brown skin and a bug-like appearance. They have four arms, two shorter upper arms, and two longer arms that are lower on their torso. The upper arms end in lobster-like claws while the lower arms appear to have three-fingered hands. They also have several shorter tentacle-like appendages both around and on their facial area.
Little else is known, but they do have the ability to shift their appearance, seemingly able to change from their true form to that of a human being at will. This disguise does appear to be rather fragile, however, as Crichton is able to tear off "Jack"'s chest accidentally. They also possess a degree of telepathy, allowing "Jack" to telepathically communicate with Crichton, but "Jack" was only able to read surface thoughts rather than completely reading his memory, to the point that he was unaware of Harvey's existence even when Harvey communicated with Crichton while he and "Jack" were talking (Although this may have been due to his current limited mental energy rather than an example of his capabilities at full strength).
How the Ancients appear in their home realm is unknown. The one unmodified Ancient Crichton encounters, "Einstein", takes a mostly human form to more easily communicate with Crichton. He differs in appearance only in his solid black eyes and pale white skin, though the latter is possible and known to occur in the human species.
The Ancients have technology and knowledge that greatly exceeds anything seen in our galaxy. They have the ability to understand and use wormhole travel and navigation. Their wormhole-based technology includes at least two wormhole weapons. In their own realm, time is subject to an Ancient's will. In our own realm, they also have the ability to create extremely realistic simulations based on a subjects' memories. These are not virtual reality, but actual duplications down to identical-seeming people and materials. These simulations are limited to only what is in the subject's memory, so when Crichton realized that everyone in "Australia" was someone he had also known in his own past in America, the Ancients' duplicity was revealed.