The Spirits were an advanced alien race that existed on PXY-887.
At some unknown point in the past, the Spirit homeworld was visited by the Goa'uld who brought their Salish slaves to the planet. The tribe were terrorized while the Spirits remained watching over them. Eventually, they fought off the Goa'uld to protect the slaves and freed them from their oppression.
The Salish praised them and gave them the name Spirits, but were never aware of their true appearance and origin; instead, they believed their mysterious benefactors to be simple guardian animal spirits. This continued for countless years as the two races shared the planet in harmony.
To this end, when SG-11 came to mine the Ke from the world, the Spirits attacked and took them prisoner. They then took on their form to infiltrate the SGC whereupon they learned that these Humans from Earth were going to mine the Ke without the consent of the Salish people. To this end, they began a process of abducting the personnel from the base with the intention of destroying the facility to end this threat.
Their mission was almost successful but SG-1 managed to appeal to the Spirits leaders. Doctor Daniel Jackson managed to appeal to T'akaya and Xe'ls who were eventually convinced to return the abducted people after their true origin was revealed to one of the Salish. The Spirits returned to their world intent on burying the Stargate to prevent future visitors to their world. (SG1: "Spirits")
The Spirits were humanoid in shape with their natural appearance resembling a human though their faces were the most distinctive feature which contained gills. Furthermore, they were shape changers allowing them to change their appearance which changed from small animals to impersonating other humans. In addition to this, they were capable of transporting people away with a gesture of their hands with the target disappearing in a white light. This was typically done with a certain hand movement but in their animal forms, a single look was capable of starting the transporter process. Finally, they had the capacity of healing one of their own though it required several of their number to perform the process.
Before their origin was revealed to the Salish, the Spirits often took on the appearance of animals in order to ensure that their presence did not disturb them. Xe'ls took on the appearance of a raven from Indian legend and T'akaya took on that of a wolf. They guided and protected the Salish and sent them pure Ke from which the humans could make their houses, weapons and other tools.