Appearance:
The Organians are a race of powerful non-corporeal beings.  The Organians stated they had once been
    humanoid, and had developed beyond the need for physical bodies millions of years earlier.
    However, they  inhabit host bodies or actually take on the appearance of humanoids themselves
    for the purposes of observing and/or interacting with humans.

    In 2154, they took over the bodies of most of the bridge crew of Enterprise at one point or another
    to observe the humans.  However, their primary hosts for this observation were Reed and Mayweather
    since they were both stationed on the bridge yet neither plays a critical role in medical emergencies.
                                                              "Observer Effect" [ENT]
    In 2267, for the purpose of interacting with the Federation and the Klingons, they took on the
    appearance of a very peaceful, friendly people living on a primitive level.  There society appeared
    to be approximately Class D-minus on Richter's scale of cultures, except for an inconsistency that
    looked to be the ruins of a fortress or castle.  Also instead of being a primitive society making progress
    toward mechanization, their society seemed to be totally stagnant with no evidence of progress for
    tens of thousands of years.  There was absolutely no advancement, and no significant change in their
    physical environment -- a laboratory specimen of an arrested culture.  Two of them then disappeared,
    first passing through a state in which they radiated intensely bright light.
                                                                "Errand of Mercy" [TOS]

Physiology:
As beings of pure thought, they are either immortal or extremely long-lived.

Homeplanet:
Organia -  In 2267 Organia IV had little of intrinsic value but it was situated in a strategic position on
    the border between the Klingon Empire and the Federation, being located on the natural invasion route.
    It is the only class M planet in the disputed area, ideally located for use by either side.
                                                             "Errand of Mercy" [TOS]

Culture:
 

As a gesture of hospitality to visitors, they create conventional points of reference -- physical bodies
    for themselves, buildings, and other objects.
Organians don't have anybody in authority.  They closest equivalent they have is a Chairman of the
    Council of Elders.  They consider interference in the affairs of others to be most disgusting, but felt
    they had no choice when they imposed the peace treaty in 2267 -- they simply could not watch idly
    as the Federation and the Klingons slaughtered each other and perhaps others. To the Organians
    violence is unthinkable.  They find the discordant emotions of less evolved beings highly painful.
                                                            "Errand of Mercy" [TOS]

Although the Organian peace treaty was occasionally mentioned in TOS, we never saw
     the Organians themselves again. It is not certain what happened to them in later years;
     the negotiations and politics portrayed in Star Trek IV and Star Trek VI suggest that
     they were no longer interfering in Federation-Klingon affairs. This is consistent with
     their declared view that such interference was most disgusting; it is possible that
     additional debate on the matter within their community diluted the will that permitted
     them to override this disgust in 2267.

History:
For ten thousand years, the Organians observed a planet that contained a silicon-based virus, and the
    reactions of species that encountered it, in an attempt to find a species with a higher level of intelligence.
    Among the species which the Organians observed as they attempted to deal with this deadly virus
    were the Klingons and the Cardassians.

2154:  After investigating a Klingon waste site on the planet with the silicon virus, a landing party
    consisting of Commander Charles Tucker and Ensign Hoshi Sato contracted the virus. Following
    their protocol of non-interferece, two Organian observers watched the crew's reaction to the
    situation by inhabiting the bodies of crewmembers, and subsequently altering their memories so
    that they had no recollection of the Organians' actions. One of the two observers (apparently the
    more senior of the two) felt that the Organians' protocols should be followed strictly; the other,
    who was impressed by the humans' compassion, felt that the time had come for the protocols to be
    altered. The senior observer initially occupied the body of Lieutenant Malcolm Reed, while his
    associate occupied the body of Ensign Travis Mayweather. All of the senior staff of the Enterprise
    were possessed by the observers at one point or another.
After a period of rapid deterioration, both Tucker and Sato died from the virus. However, at the
moment of Tucker's death, the junior Organian inhabited his body and communicated his admiration
to Captain Jonathan Archer, who had become infected himself in an effort to save his crewmen. He
explained his mission to Archer, who pled for the Organians to save his crewmembers — to
experience compassion, not merely observe it. Although it was a violation of their protocols, the
Organians complied and revived Tucker and Sato, as well as eliminating the virus from their systems
and Archer's. The crew of the Enterprise NX-01 retained no memory of their interactions with the
Organian observers.                              "Observer Effect" [ENT]

2267 (stardate 3198.4):  Fearing a surprise attack by the Klingons, Starfleet Command ordered the
    Enterprise to proceed to Organia and take whatever steps were necessary to prevent the Klingons
    from using it as a base.  While on route to Organia, war with the Klingons was officially declared.
    Captain James T. Kirk and his first officer, Spock, beamed down to the planet and attempted to
    convince the apparently-primitive Organians to accept Federation protection. The Organians seemed
    completely unconcerned with external affairs, almost to the point of apathy.  Kirk and Spock became
    stranded on the planet when a Klingon occupying force beamed down to the planet.
                                                            "Errand of Mercy" [TOS]

2267 (stardate 3201.7):  It was only when Kirk and Spock began conducting a guerilla war against the
    Klingon occupation army that the Organians abandoned their false humanoid forms and intervened,
    forcing an end to the interstellar war. Collectively, the Organians were able to neutralize Starfleet
    and the Klingon fleet, wherever they were.  They imposed the Treaty of Organia, establishing a
    framework for interaction between the two enemies, predicting that, in time, they would eventually
    become friends.                                    "Errand of Mercy" [TOS]

Individuals of Note:
Ayelborne -  Chairman of the Council of Elders.  He was able to move through a crowded citadel without
    attracting the attention of anyone there. He was able to prevent the Klingons from returning to the
    Council chamber to search for Kirk and Spock, whom he had rescued from the citadel. He was able to
    appear in at least three places light years apart at the same time, these being his own world, Earth,
    and Qo'noS.                                           "Errand of Mercy" [TOS]

Claymare -  commented that no one had died on Organia in "uncounted thousands of years".
                                                                "Errand of Mercy" [TOS]

Trefayne -  aware of events at great distances, reporting that space vehicles had assumed orbit about his
    world and that men had beamed down.
                                                                "Errand of Mercy" [TOS]