The letter K comes from the letter Κ (kappa), which was taken from the Semitic kap, the symbol for an open hand. This, in turn, was likely adapted by Semites who had lived in Egypt from the hieroglyph for "hand" representing D in the Egyptian word for hand, d-r-t. The Semites evidently assigned it the sound value /k/ instead, because their word for hand started with that sound.
In the earliest Latin inscriptions, the letters C, K and Q were all used to represent the sounds /k/ and /g/ (which were not differentiated in writing). Of these, Q was used to represent /k/ or /g/ before a rounded vowel, K before /a/, and C elsewhere. Later, the use of C and its variant G replaced most usages of K and Q. K survived only in a few fossilized forms such as Kalendae, "the calends".
When Greek words were taken into Latin, the Kappa was changed to C, with a few exceptions such as the praenomen 'Kaeso'. Some words from other alphabets were also transliterated with C. Hence, the Romance languages have 'K' only in words from other language groups. The Celtic languages also chose 'C' over 'K', and this influence carried over into Old English. Today, English is the only Germanic language to productively use hard C in addition to K (although Dutch uses it in learned words of Latin origin and follows the same hard/soft distinction in such words).
Some English linguists prefer to reverse the Latin transliteration process for proper names in Greek, spelling Hecate as "Hekate", for example. And the writing down of languages that do not have their own alphabet with the Latin one has resulted in a standardization of the letter for this sound, as in 'Kwakiutl'.
In the International Phonetic Alphabet, [k] is the symbol for the voiceless velar plosive.
Several other alphabets use characters with sharp angles to indicate the sound /k/ or syllables that start with a /k/, for example: Arabic 'ك', Hebrew 'כ' or 'ק', Korean 'ㄱ'. This kind of phonetic-visual association was studied by Wolfgang Köhler. However, there are also many examples of rounded letters for /k/, like 'క' in Telugu, 'ก' and 'ค' in Thai, 'Ք' in Armenian, 'ክ' in Geez, and 'C' in Latin.
In modern-day English slang, the word "k" is used as a substitute for the abbreviation "OK", or Okay. In International Morse code it is used to mean "over". Similarly, in colloquial English writing, the prefix kilo-, meaning one thousand, can be abbreviated to "k".