Contrary to the musings of some, "revenge is a dish best served cold" is not a Klingon proverb, but it is the sort of thing that they would say. The archetypical warriors, Klingons hail from the logically-named Klingon homeworld, more properly known as Qo'noS, and are the only species of influence within the borders of the Klingon Empire. Life in Klingon society is appropriately nasty, brutish, and short, bearing a resemblence (one that no Klingon would ever admit) to the ancient Earth society of Sparta, with battle frequently determining promotion, and an officer being forced to be constantly on their toes to survive and to prosper.
Derogatarily known as 'ridgeheads', this epithet refers to one of the distinctive features natural to most Klingons, a series of strong, boney ridges along a Klingon's forehead. In general, Klingons are among the tallest and sturdiest-looking creatures one will find in the Alpha Quadrant, and their physical abilities live up to their impressive appearance. Referring to the distinctive ridges as 'flavour curls' or something similar in a Klingon's presence will likely get at least some strong profanity hurled in one's direction, and possibly something more fatal.
Like many of their be-muscled brethren, Klingons are on average a trifle slow on the uptake. However, most Klingons can hardly be called stupid and their physical abilities are certainly impressive, capable of performing great feats of strength, dexterity, and endurance. This makes a Klingon an extremely grave foe in hand-to-hand combat.