The Klingons are warrior race noted for both brutality and their code of honor. Traditionally seeing other races as potential enemies, their relations with their neighbors are tenuous at best, and often quickly break down into war. Repeated efforts by the Federation have slowly gotten them to try means of interacting with other races other than shooting at them.
The Klingons were unified under Kahless the Unforgettable two millennia ago, and apparently came into contact with Vulcans and humans before the founding of the Federation. The source of the information on the first contacts are suspect, however, as the Enterprise - NX01 is stated as taking only five days to reach the Klingon homeworld from Earth at a time when Warp 4 was considered fast. Stranger still, Klingons in the bulk of recorded history have been humanoids larger on average than humans with ridged foreheads, but those encountered around the time of Captain Kirk looked human. It is unclear how this happened, or what happened to the "human Klingons" as this appearance among them disappeared without explanation. The Klingons acknowledge this appearance existed, but refuse to talk about it.
Klingons follow a code of honor, considering it dishonorable to lie, cheat, steal, or bluff. While they do take prisoners (sometimes), they do not hold hostages. They pride themselves as warriors, and actually prefer to fight with traditional melee weapons over energy weapons. The "Human Klingons," in contrast, seem to have had little honor, frequently using assassination and other backstabbing to advance in rank, took and executed hostages, tourtured prisoners, and using weapons Klingons before and after would find dishonorable, such as poison gas. They did not have the preference for bladed weapons.
Klingons train to hunt from an early age, and prefer their meat raw. Needless to say, Japanese restaurants are quite popular with Klingons frequenting human areas. One of the more famous Klingon dishes is a plate of large worms. In earlier times, it went by several names. Then one day, a Klingon chef put a plate of it in front of a human, whom reacted by exclaiming, "GAH!!" The name stuck.
Klingon ships are heavily armed with less powerful shields than ships of other races. They are also quite spartan (A running joke among Terrans is that Klingon ships have no toilets, to which one Klingon answered that Terran media showed no sign of them in Federation vessels either) . Apparently, the only recreation on Klingon ships, other than combat practice, is companionship with crew members of the opposite sex. This may explain why the Empire never runs out of Klingons.
Virtually all Klingons have a deep dislike of the cute, cuddly Tribble. Tribbles are banned from trade or transport in Klingon territory. Nine out of ten xenoculturalists are puzzled as to why. The tenth had seven kids whom were almost always playing recordings of "Barney" in his home. *Footnote - a surprise attack on an outpost by Klingons was foiled when the first building they stormed happened to have a children's birthday party with one man entertaining the kids in a "Barney" suit. The Klingons turned and fled, screaming. *
Captian "Treki" Riker of the USS Freedom - D and E , was half-Klingon.