Star Trek Canon Special Note
Not everything listed in the origin is Canon. That is because only television shows and movies are considered canon for Star Trek. This being a comic website there are many things here that are not canon. So unless it is contradicted in film it should be listed as part of the team biography.
Klingons are a warrior race on Qo'nos (pronounced Kronos). To survive the violence of their harsh home-world, they have evolved highly redundant body systems, including an eight-chambered heart, 2 livers, 4 kidneys. They also (normally) have a dense protective ridge of bone over their skulls.
The Klingons are a very conservative, honor-bound culture, ruled by a hereditary warrior class. This is thought to be a result of the events which led them to become a starfaring race. Four hundred years before the formation of the Federation Klingons were ruled by Kahless the Unconquerable. Kahless was a mighty warrior and was revered as a Herculean figure in the Klingon culture.
Later an alien species known only as the Hur'q (outsiders) tried to conquer and enslave Qo'nos during the waning days of the Empire of Kahless, only to find that Klingons make poor slaves. They rebelled, stole the Hur'q technology, and over the next thousand years used it to conquer first the rest of Klingonkind, then other worlds and races. The Empire had come to the stars and at it's height they had conquered over 750 class M worlds.
The Klingon Empire encountered Humans in the mid-2050s, leading to tense relations and minor conflicts that would, a century later, lead to open war between the two. During their early contacts, the Klingons experimented with a genetic virus which they hoped would give them enhanced attributes, based on the DNA of human Augments. However, this virus proved contagious and fatal, and spread rapidly to a large population. The virus was eventually cured, although the side-effects of the curse left many Klingons with human-like ridgeless foreheads and minor neural alterations that would not be fully cured for generations.
When the Klingons and the human-founded Federation began to expand into the same regions of space, it led to escalating conflicts which broke into open war in 2267. However, the powerful race of energy beings known as the Organians intervened and forced a cease-fire; the Organian Peace Treaty was negotiated by both sides under duress. The Klingons occasionally tried to get around this treaty by means ranging from subtlety introducing new weapons technology to the natives, up to mass poisoning of civilian populations. The Klingons also forged a temporary alliance with the secretive Romulan Empire.
As an outlier alone in a world of human's a Klingon was adopted by a family of human's and raised on Earth. His name was Worf and he became the first Klingon in Starfleet. Worf would spend much of his life between the two cultures and eventually have a hand in selecting the new Canceler of the Empire after killing Gowron the then current Chancellor.
Eventually, the Klingons found the Romulans too manipulative and untrustworthy, and the Klingon Empire and the Federation found common ground opposing their mutual enemy. They forged an accord at Khitomer, the site of the greatest Romulan betrayal, although the alliance was not always strong; during the buildup to the Dominion War, it was broken entirely, and the Federation once again fought the Klingons. However, once again, a common enemy would force the two cultures to work together as allies.
Klingons were created during the first series of the Star Trek television show. Originally they looked more human due to the cost of special effects. It was not until the first Star Trek movie that Klingons had ridges on their foreheads, but even then they were all the same. It was later that the ridges became unique to each Klingon.