Klingons are a humanoid species from the planet Qo'noS. They are known primarily as a race of warriors and soldiers and for their honour-based culture. By the 24th century, they were a major power in their region.
One of the most important events in Klingon history was the rise of Kahless the Unforgettable during the 9th century. Kahless slew Molor, a tyrant that ruled over the Klingons, and became emperor forming the Klingon Empire. As the emperor he introduced a set of codes of honour that would shape the Klingons for centuries to come. (VOY: "Day of Honor")
During the 14th century the Klingons were conquered by the Hur'q. The Klingons were eventually freed of the Hur'q, who would all but disappear. One of the artefact pillaged by the Hur'q was the Sword of Kahless, a legendary weapon that was considered to be the first bat'leth. (DS9: "The Sword of Kahless")
In 2154, while experimenting with Human augment DNA the Klingons accidentally created a plague that resulted in their facial ridges dissolving, an increase in strength and intelligence of Klingons and then caused death. With the assistance of Phlox the Klingons were able to halt the plague after it dissolved the ridges leaving millions of Klingons with a far more human appearance that was also passed on to their offspring. The event was considered embarrassing by the Klingons and they did not discuss the event with non-Klingons. (ENT: "Affliction", "Divergence"; DS9: "Trials and Tribble-ations")
During the 23rd century, the Klingons and Federation clashed but by the end of the century the two powers entered a prolonged period of peace. (Star Trek: The Original Series; Star Trek VI: The Undiscovered Country)
By the late-24th century, Klingons had begun to become displeased with their relatively peaceful state and support for a return to the "old ways" grew. This sentiment eventually helped spur on the Klingon-Cardassian War. (DS9: "The Way of the Warrior")
They believed in an afterlife. Honourable warriors, including non-Klingons, after they died would go to Sto-Vo-Kor were they would be with Kahless and fight an eternal battle. Alternatively, the dishonoured would go via the Barge of the Dead to Gre'thor. (DS9: "Shadows and Symbols"; VOY: "Barge of the Dead")
Kahless was revered as an almost religious figure and many attempted to emulate him. It was believed he would one day return to lead the Empire. A group of monks fulfilled this prophesy when they created a clone of Kahless. However, it was exposed as a clone and instead of leading the empire the clone was given a purely ceremonial role as the emperor and actually wielded no power at all. (TNG: "Rightful Heir")